nfc: s3fwrn5: use signed integer for parsing GPIO numbers

GPIOs - as returned by of_get_named_gpio() and used by the gpiolib - are
signed integers, where negative number indicates error.  The return
value of of_get_named_gpio() should not be assigned to an unsigned int
because in case of !CONFIG_GPIOLIB such number would be a valid GPIO.

Fixes: c04c674f

 ("nfc: s3fwrn5: Add driver for Samsung S3FWRN5 NFC Chip")
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201123162351.209100-1-krzk@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>