ath9k: detect more kinds of invalid descriptors

If AR_CRCErr, AR_PHYErr, AR_DecryptCRCErr or AR_MichaelErr is indicated
in the rx status word, but AR_RxFrameOK is also set, the descriptor
contents are typically invalid. This can show up as a warning about
invalid MCS rates in a frame. Even with those checks in place, a
descriptor with invalid MCS rates can still sometimes make it through to
the driver (mostly on older hardware like AR91xx).

Detect such errors in the last descriptor of a frame and discard the
whole frame if present.

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