ath9k_hw: fix dma descriptor rx error bit parsing

An Rx DMA descriptor can have multiple error bits set, and some error
bits (e.g. MIC failure) are filtered by the driver based on other criteria.
Remove the 'else' in various error bit checks so that all error information
is properly passed to the driver.

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