ath9k: decouple RX error checking for DFS

Previous RX error checking was done exclusive-or for different error
types and caused DFS pulse events to be dropped when other error
flags (e.g. CRC) were set simultaneously.

This patch decouples PHY error processing from other types and ensures
that all pulses detected by HW are accounted by the pattern detector.

Signed-off-by: Zefir Kurtisi <zefir.kurtisi@neratec.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>