net: dsa: fix led_classdev build errors

Fix build errors when LEDS_CLASS=m and NET_DSA_HIRSCHMANN_HELLCREEK=y.
This limits the latter to =m when LEDS_CLASS=m.

microblaze-linux-ld: drivers/net/dsa/hirschmann/hellcreek_ptp.o: in function `hellcreek_ptp_setup':
(.text+0xf80): undefined reference to `led_classdev_register_ext'
microblaze-linux-ld: (.text+0xf94): undefined reference to `led_classdev_register_ext'
microblaze-linux-ld: drivers/net/dsa/hirschmann/hellcreek_ptp.o: in function `hellcreek_ptp_free':
(.text+0x1018): undefined reference to `led_classdev_unregister'
microblaze-linux-ld: (.text+0x1024): undefined reference to `led_classdev_unregister'

Signed-off-by: Randy Dunlap <[email protected]>
Reported-by: kernel test robot <[email protected]>
Link: lore.kernel.org/r/[email protected]
Cc: Kurt Kanzenbach <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/net/dsa/hirschmann/Kconfig

@@ -4,6 +4,7 @@ config NET_DSA_HIRSCHMANN_HELLCREEK
 	depends on HAS_IOMEM
 	depends on NET_DSA
 	depends on PTP_1588_CLOCK
+	depends on LEDS_CLASS
 	select NET_DSA_TAG_HELLCREEK
 	help
 	  This driver adds support for Hirschmann Hellcreek TSN switches.