ath5k: Extend get_default_sifs/slot_time

 * Extend get_default_sifs/slot_time to include timings for turbo
 half and quarter rate modes.

 * AR5210 code for now uses timings already on core clock units
 instead of usecs so rename them (we 'll clean it up later).

Signed-off-by: Nick Kossifidis <mickflemm@gmail.com>

Signed-off-by: John W. Linville <linville@tuxdriver.com>