ath5k: Update RF Buffer handling

 * Use the new way to modify rf buffer and put some rf buffer
   documentation on rfbufer.h

 * Merge all rf regs functions to one

 * Sync with legacy HAL and Sam's HAL

 * Set gain_F settings so that gain_F optimization engine works
   on RF5111/RF5112 (note that both HALs only use step 0 for RF5111
   and they don't use gain_F optimization for this chip, code is
   there but is never used)

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