rt2800: refactor RFCSR initialization

rt2800_init_rfcsr() is too big, split RF initialization into per chip
functions. Code will change, but we should setup the same values onto
RF registers and in the same order as before.
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>