Input: sh_keysc - factor out hw access functions

Update the sh_keysc driver to factor out the register access functions
sh_keysc_read(), sh_keysc_write() together with sh_keysc_level_mode().
This makes the code a bit easier to follow.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>