Input: max8925_onkey - avoid accessing input device too early

Input device must be allocated (but not necessarily registered) before
requesting IRQs, otherwise there is a chance that IRQ handler fires and
tries to reference not yet allocated input device.

Also it makes sense to store relative IRQ numbers in max8925_onkey_info
structure as they are needed in suspend/resume which we expect to be
called more often than probe and remove.

Acked-by: Haojian Zhuang <haojian.zhuang@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>