ath5k: Set turbo bit on rf bank 2

 * A diff between rfbuffer settings of turbo and non-turbo
 modes indicates there is a bit on bank 2 related to turbo operation
 (it's set on turbo modes). This bit is present on all radios except
 RF5413 that seems to have a completely different bank 2. Also
 since 2317 has the same rf-registers locations with 2425 and
 since the bit exists on 2317 I assume it also exists on 2425/2417).
 So in case we use turbo mode (40MHz) enable it on bank modification.

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