brcmfmac: fix potential NULL pointer dereference in brcmf_fws_flow_control_check()

The dereference to 'ifp' in debug code should be moved below the NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>