ath5k: use direct function calls for descriptors when possible

Use direct function calls for ath5k_hw_setup_rx_desc() and
ath5k_hw_setup_mrr_tx_desc() instead of a function pointer which always pointed
to the same function in the case of ath5k_hw_setup_rx_desc() and which is
easily unified in the case of ath5k_hw_setup_mrr_tx_desc().

Also simplify the initialization function for the remaining function pointers.

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