ath9k: Fix error condition for corrupt descriptors

In case a descriptor has the "done" bit clear and the
next descriptor has it set, we drop both of them. If
the packet that is received after these two packets
is dropped for some reason, "discard_next" will not cleared.
Fix this.

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