ath9k: use consistent value for REDUCE_SCALED_POWER_BY_THREE_CHAIN

The REDUCE_SCALED_POWER_BY_THREE_CHAIN symbol is
defined in different eeprom files, and the value
varies between the different files.

In eeprom_def.c and in ar9003_eeprom.c the value
of the symbol is 9, however the comments in these
files indicates the value should be 10*log10(3)*2
which is 9.54242509439325. Replace the the value
to 10 in these files.

Also add comments to eeprom_9287.c.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>