ath9k: Handle corrupt descriptors properly

The MIC/PHYERR/CRC error bits are valid only for
the last desc. for chained packets. Check this early
in the preprocess() routine and bail out.

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>