sysfs: make frequency error messages more actionable

Print which part of the range is exceeded (lower or higher bound), and give hint
about using the physic constant only on the lower end.

host/sysfs/i2c.go

@@ -129,8 +129,11 @@ func (i *I2C) Tx(addr uint16, w, r []byte) error {
 
 // SetSpeed implements i2c.Bus.
 func (i *I2C) SetSpeed(f physic.Frequency) error {
-	if f < 100*physic.KiloHertz || f > 10*physic.MegaHertz {
-		return fmt.Errorf("sysfs-i2c: invalid speed %s; did you forget to multiply by physic.KiloHertz?", f)
+	if f > 100*physic.MegaHertz {
+		return fmt.Errorf("sysfs-i2c: invalid speed %s; maximum supported clock is 100MHz", f)
+	}
+	if f < physic.KiloHertz {
+		return fmt.Errorf("sysfs-i2c: invalid speed %s; minimum supported clock is 1KHz; did you forget to multiply by physic.KiloHertz?", f)
 	}
 	drvI2C.mu.Lock()
 	defer drvI2C.mu.Unlock()

host/sysfs/spi.go

@@ -92,8 +92,11 @@ func (s *SPI) String() string {
 
 // LimitSpeed implements spi.ConnCloser.
 func (s *SPI) LimitSpeed(f physic.Frequency) error {
-	if f < 100*physic.Hertz || f > physic.GigaHertz {
-		return fmt.Errorf("sysfs-spi: invalid speed %s; did you forget to multiply by physic.MegaHertz?", f)
+	if f > physic.GigaHertz {
+		return fmt.Errorf("sysfs-spi: invalid speed %s; maximum supported clock is 1GHz", f)
+	}
+	if f < 100*physic.Hertz {
+		return fmt.Errorf("sysfs-spi: invalid speed %s; minimum supported clock is 100Hz; did you forget to multiply by physic.MegaHertz?", f)
 	}
 	s.Lock()
 	defer s.Unlock()
@@ -105,8 +108,11 @@ func (s *SPI) LimitSpeed(f physic.Frequency) error {
 //
 // It must be called before any I/O.
 func (s *SPI) Connect(f physic.Frequency, mode spi.Mode, bits int) (spi.Conn, error) {
-	if f < 100*physic.Hertz || f > physic.GigaHertz {
-		return nil, fmt.Errorf("sysfs-spi: invalid speed %s; did you forget to multiply by physic.MegaHertz?", f)
+	if f > physic.GigaHertz {
+		return nil, fmt.Errorf("sysfs-spi: invalid speed %s; maximum supported clock is 1GHz", f)
+	}
+	if f < 100*physic.Hertz {
+		return nil, fmt.Errorf("sysfs-spi: invalid speed %s; minimum supported clock is 100Hz; did you forget to multiply by physic.MegaHertz?", f)
 	}
 	if mode&^(spi.Mode3|spi.HalfDuplex|spi.NoCS|spi.LSBFirst) != 0 {
 		return nil, fmt.Errorf("sysfs-spi: invalid mode %v", mode)