To enable eventual removal of pr_warning

This makes pr_warn use consistent for drivers/input

Prior to this patch, there were 8 uses of pr_warning and
17 uses of pr_warn in drivers/input
Signed-off-by: NJoe Perches <joe@perches.com>
Signed-off-by: NDmitry Torokhov <dmitry.torokhov@gmail.com>