brcmfmac: fix potential NULL dereference

Fix a samtch warnings catched by Fengguang's 0-DAY system:
+ drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.c:3572 brcmf_cfg80211_sched_scan_start() error: we previously assumed 'request' could be null (see line 3571)

Cc: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Acked-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>