ath5k: Remove RF5413 from rf gain optimization functions

 * Since RF2413 it seems that RF_BUFFER settings are different (notice
that the last part -"bank 7" or whatever- is smaller than in 5111/5112).
So until we know what's going on we assume there is no gain optimization
stuff in post-5112 chips.

Changes-licensed-under: ISC
Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@winlab.rutgers.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>