ath5k: Update EEPROM code

*Read misc2...6 values from eeprom since we want to use them (fixes
 wrong power calibration info offset on RF2413+ chips)

*Initialize num_piers to 0 for RF2413 chips (note that we read 2GHz
 frequency piers while reading mode sections, we have to ignore them
 -usualy they are 0xff anyway but during my tests i got a 1 on b mode
 with no data- and use the newer eemap.

*Add some more comments (please forgive my poor English ;-( ) and
 some minor code cleanup

*Tested on 2425 and 2112 and has the same data with ath_info (i
 wrote some debug code on debug.c to print everything like ath_info
 but i haven't tested it yet on 5111 and it's full of > 80 col lines,
 if anyone wants to play with it let me know).
Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>