ath9k_hw: clean up ath9k_hw_setuprxdesc

The ATH9K_HW_CAP_AUTOSLEEP check is bogus, the rx status area needs to be
cleared on all non-EDMA PCI/AHB based chipsets anyway.
Limit the memset to the rx status area to improve performance.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>