mac80211: fix key replacing, hw accel

Even though I thought about it a lot and had also tested it, some
of my recent changes in the key code broke replacing keys, making
the kernel oops because a key is removed from a list while not on
it.

This patch fixes that using the list as an indication whether or
not the key is on it (an empty list means it's not on any list.)

Also, this patch fixes hw accel enabling, the check for not doing
hw accel when the interface is down was lost and is restored by
this.

Additionally, move adding the key to the list into the function
__ieee80211_key_replace() for more consistency.

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