ath5k: clarify logic when to enable spur mitigation filter

The old code logically did not make sense and seems to have been confused by
the fact that we could have newer EEPROMs on older hardware. In any case the
spur mitigation filter was set if the srev was >= AR5K_SREV_AR5424.

Spur info is available only from EEPROM versions bigger than 5.3 but but the
EEPOM routines will use static values for older versions, so that should be
o.k.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>