mac80211: fix deadlocks in debugfs_netdev.c

The bug shows up with CONFIG_PREEMPT enabled. Pointed out by Andrew Morton.

Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Luis Carlos Cobo <luisca@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>