rt2x00: rt2800lib: fix frequency offset boundary calculation

The current code in the 'rt2800_adjust_freq_offset'
function limits the device specific frequency offset
value to FREQ_BOUND but ignores the fact that the
uppermost bit is not part of the frequency offset
value. As the result, the driver always uses the
FREQ_BOUND value if the uppermost bit is set.

Update the code to use the correct source value
for calculating the boundary.

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>