rt2x00: change function pointers for register accessors

This prepares the driver for changing all the 'read' register accessors
to return the value instead of passing it by reference. Since a lot
of them are used in callbacks, this takes care of the callbacks first,
adding a couple of helpers that will be removed again one at a time.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>