brcmfmac: use memset when setting a broadcast mac address

The driver had a global constant ether_bcast, which was copied
whenever a broadcast mac address was needed. This patch does a
memset(dest, 0xFF, ETH_ALEN) instead and consequently removes
the global ether_bcast.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Franky Lin <frankyl@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>