ath5k: PHY code cleanup

 * Clean up initial rf buffer settings (new file rfbufer.h) and introduce a
   new way to access specific rf registers (will use it later)

 * Clean up initial rf gain settings by moving them on a new file (rfgain.h)
   so we can later work on gain optimization functions

 * Update initial rf buffer settings and initial rf gain settings from HALs.
   This breaks things for now because our current dumps come from pre-configured
   rf buffer (regdumps already had the needed values set from binary HAL).
Signed-off-by: NNick Kossifidis <mickflemm@gmail.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>