rt2x00: Check for 5GHz band in link tuner

Fix a typo in the link tuner where accidently the
2GHz band was checked instead of the 5GHz band.
This forced the link tuner to work in an invalid
range for the currently active band.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>