brcmfmac: increment hard_header_len instead of overriding

In brcmf_net_attach() the hard_header_len is set to sum of ETH_HLEN
and the headroom needed by the bus interface. Better use increment
instead as hard_header_len is already initialized upon alloc_netdev().

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