ath5k: fix wrong max power per rate eeprom reads for 802.11a

This patch reduces the per rate target power eeprom reads for
AR5K_EEPROM_MODE_11A from 10 to 8, as there are only 8 valid
power curve entries on the eeprom. The former 10 reads lead to
equal max power limits per rate and this causes an increasing
distortion for all rates above 24 MBit and leads to a needless
poor performance in 802.11a mode.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>