rt2x00: rt2800lib: optimize frequency offset adjustment

Don't write the new value into the register if it is
the same as the old value to avoid unncessary USB bus
traffic with USB devices. The change also saves a few
cycle on MMIO based devices.

Based on the DPO_RT5572_LinuxSTA_2.6.0.1_20120629
driver.

Reference:
  RTMPAdjustFrequencyOffset function in common/rt_rf.c

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>