ath5k: Minor EEPROM documentation updates

Here are some minor updates for EEPROM, mostly documentation and some small
fixes which have no effect at the moment.

- fixed_bias is not available for B mode.

- AR5K_EEPROM_[RT]X_CHAIN_DIS is 3 bit. this is MIMO and will not be used in
  ath5k, but just to be correct.

- AR5K_EEPROM_JAP_MID_EN added, and shift of following flags adapted.

- added some documentation for EEPROM values and some comments.

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