ath5k: Clean up turbo mode initvals/rfregs

 * Clean up what's left of turbo mode, since we handle all
 register modifications (rfbuffer comes next) on code there
 is no need to have duplicated arrays.

 * Rename change_channel to skip_pcu on initvals.c as we did
 on reset.c

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