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>