ath5k: Simplify cw_min/max and AIFS configuration

Get rid of overly complicated cw_min/max and AIFS configuration:

* Validate values in ath5k_hw_set_tx_queueprops(), so we can use them directly
  without further checks or computation in ath5k_hw_reset_tx_queue().

* Simplifiy by using AR5K_TUNE_AIFS|CWMIN|CWMAX variables directly since we
  don't support XR or B channels. That way we can also remove
  AR5K_TXQ_USEDEFAULT and the confusing logic around it.

* Update data types: AIFS is u8, CW's are u16.

* Remove now unneeded variables in ath5k_hw.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>