staging: panel: fix sparse warnings in keypad_read

This patch fixes two sparse warnings related to keypad_read :
warning: incorrect type in argument 1 (different address spaces)
warning: incorrect type in initializer (incompatible argument 2 (different address spaces))

Signed-off-by: Bastien Armand <armand.bastien@laposte.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>