pinctrl: remove double pin validity check.

Function pin_is_valid just call pin_desc_get which is in pin_request
call some line below. Remove pin_is_valid() check.

Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Marek Belisko <marek.belisko@open-nandra.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>