ath9k/mac80211: disallow fragmentation in ath9k, report to userspace

As I've reported, ath9k currently fails utterly when fragmentation
is enabled. This makes ath9k "support" hardware fragmentation by
not supporting fragmentation at all to avoid the double-free issue.
The patch also changes mac80211 to report errors from the driver
operation to userspace.

That hack in ath9k should be removed once the rate control algorithm
it has is fixed, and we can at that time consider removing the hw
fragmentation support entirely since it's not used by any driver.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>