staging: mt7621-gpio: avoid including 'gpio.h'

Including file '<linux/gpio.h>' should be avoided in
new drivers code, so just remove it because it is
no necessary at all.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>