rt2x00: Remove debugfs CSR access wrappers

Move calculation of CSR register offset into rt2x00debug.c
and remove the wrapper functions from each individual driver.
(Except rt2500usb, which still needs to wrap for the
different value type argument).

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>