ath5k: More EEPROM code updates

* Don't scale power values on RF5111 EEPROMs because they get out of
  bounds (power is u8, so multiplying power by 50 is too much and there
  is no reason to do so -we don't do it on other chips anyway-). HAL
  does it as a technique to handle 0.5 dbm steps but i believe it's not
  the right thing to do and certainly not the right place to do it. We
  'll work this out on interpolation code for all chips (0.5 or 0.25
  steps etc) in a generic way.

Signed-Off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>