rt2x00: rt2800lib: use step-by-step frequency offset adjustment on MMIO devices
According to the DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver, the RFCSR17 register can't be programmed in
one step on devices which are using the frequency
offset adjustment code.
Update the code to use step-by-step adjustment.
Reference:
RT30xxWriteRFRegister function in common/rt_rf.c
Signed-off-by: Gabor Juhos <juhosg@openwrt.org> Signed-off-by: John W. Linville <linville@tuxdriver.com>