ath5k: Read Spur channels from EEPROM

* Read Spur channel information from EEPROM and use default channels
 for RF5413 compatible chips that don't have this info on EEPROM.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Bob Copeland <me@bobcopeland.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>