staging: fbtft: array underflow in fbtft_request_gpios_match()

"val" can be negative, so we'd write before the start of the
par->gpio.db[] array.

Fixes: c296d5f9 ("staging: fbtft: core support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>