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