[MAC80211]: remove/change some comments about Michael MIC hardware offload

There are a few TODO comments in the mac80211 sources regarding
hardware offload for Michael MIC verification. Those items are,
however, better handled in the driver instead of the stack, if
any device requires such hand-holding.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>