rt2x00: rt2800lib: move rt2800_adjust_freq_offset function

Move the rt2800_adjust_freq_offset function before
the channel configuration functions to make it usable
from those without a forward declaration.

The patch contains no functional changes.

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