brcmfmac: fix memory leakage in msgbuf

brcmfmac: fix memory leakage in msgbuf

The kbuild robot came up with the following warning:

tree:   .../kernel/git/linville/wireless-next.git master
head:   dc6be9f54a4ecb0a09765d1f515ed947d86b7528
commit: 9a1bb60250d2b6b546a62e5b73f55c4f1d22016b
	 [5/13] brcmfmac: Adding msgbuf protocol.

coccinelle warnings:
 drivers/net/wireless/brcm80211/brcmfmac/msgbuf.c:1309:1-28:
   alloc with no test, possible model on line 1318

Looking into the issue, it turned out that the referred allocation
buffer was not being released in failure path nor upon module
unload.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>