ath5k: Set all IFS intervals, not just slot time

 * Replace set_slot_time with set_ifs_intervals that also sets
 the various inter-frame space intervals based on current bwmode.

 * Clean up AR5210 mess from reset_tx_queue, AR5210 only has one
 data queue and we set IFS intervals for that queue on set_ifs_intervals
 so there is nothing left to do for 5210 on reset_tx_queue.

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