brcmfmac: fix NULL pointer access in brcmf_fweh_detach()

brcmf_fweh_detach can be called while ifp is already NULL, due to
init error. Fix NULL pointer access by checking ifp.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>