rt2x00: rt2800lib: use a MCU command for frequency adjustment on USB devices

According to the Ralink driver, there is an MCU
command which can be used to send the frequency
offset value directly to the USB device without
going through the RFCSR writing sequence.

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>