ath9k: fix spur mitigation no-spur case for AR9002

For the AR9002, the spur frequency read from the EEPROM is mangled
before being compared against AR_NO_SPUR.  This results in the driver
trying to set up the spur mitigation for bogus spurs, rather than
cleanly breaking out.

Signed-off-by: Brian Prodoehl <bprodoehl@nomadio.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>