ath5k: Use helper function to get eeprom mode from channel

Introduce a helper function to get the EEPROM mode from channel and remove
multiple similar switch statements. Also since it's now easy to get the EEPROM
mode from the channel, use them inside the functions which need it, instead of
passing a redundant ee_mode parameter.

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