ath5k: Small cleanup on tweak_initvals

 * Now that we properly set rx/tx latencies for AR5311 remove
 that old buggy part of code left inside ath5k_hw_tweak_initval_settings
 that was never executed (you can't have an RF5112 radio on a mac older
 than AR5212). Also use a magic value for 5311 PHY_SCAL value.

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