brcmfmac: remove null-pointer check in .sched_scan_start() callback

In brcmf_cfg80211_sched_scan_start() the request parameter was checked
for being non-null. However, it never is so remove the check which gets
rid of following smatch warning:

drivers/net/wireless/brcm80211/brcmfmac/wl_cfg80211.c:2904
brcmf_cfg80211_sched_scan_start()
warn: variable dereferenced before check 'request' (see line 2897)

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Piotr Haber <phaber@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>