staging: ste_rmi4: remove gpio.h include

gpio.h is not needed and is going to be removed.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>