Input: gpio_keys - use of_property_read_u32()

Use the of_property_read_u32() helper function to retrieve u32 values
from the device tree. Also do not pass the len parameter to
of_get_property if it isn't checked afterwards.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>