ath5k: Check EEPROM before tweaking SERDES

* Read PCI-E infos offset from EEPROM and if it points to
   serdes section (0x40), enable serdes programming (further tweaking
   of serdes values during attach). This follows Legacy and Sam's
   HAL sources.

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Acked-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>