ath5k: Clean up eeprom parsing and add missing calibration data

This patch brings the ath5k eeprom parsing code in sync with the work
done on ath_info by Nick Kossifidis and integrates the missing parts
based on the code of the Atheros Legacy HAL release.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>