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

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: 781a07da9bb9 ("Input: ads7846 - add dummy command register clearing cycle")
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20241111-input-ads7846-increase-xfer-array-size-v1-1-06cd92e9f20f@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>