ath9k: Fix AR9287 calibration

The AR9287 calibration code was not being called because of an
incorrect MAC revision check.
This forced the AR9287 to use the AR9285 initial calibration code and
bypass the AR9287 code entirely.

Signed-off-by: Adrian Chadd <adrian@freebsd.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>