gpio: mb86s70: Revert "Return error if requesting an already assigned gpio"

Commit fd9c963c ("gpio: mb86s70: Return error if requesting an
already assigned gpio") adds code that infers from the state of the
GPIO Pin Function Register (PFR) whether a GPIO has been assigned
already. This assumes that the pin functions are set to 'peripheral'
when the driver is loaded, which is not guaranteed. Also, the GPIO
layer is perfectly capable of keeping track of which GPIOs have been
assigned already, so we shouldn't need this check in the first place.

This reverts commit fd9c963c

.

Cc: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>