USB: gadget: pxa25x uses gpio_is_valid

Use gpio_is_valid instead of assuming that every GPIO
number != 0 is valid while 0 is not.

Signed-off-by: Philipp Zabel <philipp.zabel@gmail.com>
Acked-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>