Input: ads7846 - increase xfer array size in 'struct ser_req'

Clang warns (or errors with CONFIG_WERROR=y, trimmed for brevity):

  drivers/input/touchscreen/ads7846.c:412:2: error: array index 6 is past the end of the array (that has type 'struct spi_transfer[6]') [-Werror,-Warray-bounds]
    412 |         req->xfer[6].tx_buf = &req->scratch;
        |         ^         ~
  ...
  drivers/input/touchscreen/ads7846.c:416:2: error: array index 7 is past the end of the array (that has type 'struct spi_transfer[6]') [-Werror,-Warray-bounds]
    416 |         req->xfer[7].rx_buf = &req->scratch;
        |         ^         ~
  drivers/input/touchscreen/ads7846.c:334:2: note: array 'xfer' declared here
    334 |         struct spi_transfer     xfer[6];
        |         ^

Increase the size of xfer in 'struct ser_req' to ensure that there is
enough space for all necessary members and there are no out of bounds
accesses, clearing up the warning.

Fixes: 781a07d ("Input: ads7846 - add dummy command register clearing cycle")
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Marek Vasut <[email protected]>
Link: https://lore.kernel.org/r/20241111-input-ads7846-increase-xfer-array-size-v1-1-06cd92e9f20f@kernel.org
Signed-off-by: Dmitry Torokhov <[email protected]>

drivers/input/touchscreen/ads7846.c

@@ -331,7 +331,7 @@ struct ser_req {
 	u8			ref_off;
 	u16			scratch;
 	struct spi_message	msg;
-	struct spi_transfer	xfer[6];
+	struct spi_transfer	xfer[8];
 	/*
 	 * DMA (thus cache coherency maintenance) requires the
 	 * transfer buffers to live in their own cache lines.