cfg80211: Fix mutex locking in reg_last_request_cell_base

should fix the following issue

	[ 3229.815012] [ BUG: lock held when returning to user space! ]
	[ 3229.815016] 3.5.0-rc7-wl #28 Tainted: G        W  O
	[ 3229.815017]
	------------------------------------------------
	[ 3229.815019] wpa_supplicant/5783 is leaving the kernel with locks still held!
	[ 3229.815022] 1 lock held by wpa_supplicant/5783:
	[ 3229.815023]  #0: (reg_mutex){+.+.+.}, at: [<fa65834d>]
	reg_last_request_cell_base+0x1d/0x60 [cfg80211]

Cc: Luis Rodriguez <mcgrof@gmail.com>
Signed-off-by: Mohammed Shafi Shajakhan <mohammed@qca.qualcomm.com>
Tested-by: Luciano Coelho <coelho@ti.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>