Input: gpio_keys - use dev_ macros to report information

The gpio_keys driver is binding to a platform device but using pr_err()
to report errors. Change to using dev_err() so that all messages are
prefixed by the device name.

Signed-off-by: Ben Dooks <ben@simtec.co.uk>
Signed-off-by: Simtec Linux Team <linux@simtec.co.uk>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>