brcmsmac: fix smatch warning found in ampdu.c

This patch fixes potential NULL pointer dereference in ampdu. This
was found running smatch static code checker. Smatch warning says:

drivers/net/wireless/brcm80211/brcmsmac/ampdu.c:741 brcms_c_sendampdu()
	 warn: variable dereferenced before check 'p'

Reported-by: Dan Carpenter <dan.carpenter@oracle.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>