wireless: convert reg_regdb_search_lock to mutex

Stanse discovered that kmalloc is being called with GFP_KERNEL while
holding this spinlock.  The spinlock can be a mutex instead, which also
enables the removal of the unlock/lock around the lock/unlock of
cfg80211_mutex and the call to set_regdom.

Reported-by: Jiri Slaby <jirislaby@gmail.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>