brcm80211: rename variable in _brcmf_set_multicast_list()

The variable allmulti was used to provision IFF_ALLMULTI to the
device as well as IFF_PROMISC. For clarity the variable has been
renamed.

Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>