gpiolib: don't dereference a desc before validation

The fix restores a proper validation of an input gpio desc, which
might be needed to deal with optional GPIOs correctly.

Fixes: 02e47980 ("gpio: Alter semantics of *raw* operations to actually be raw")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>