brcmfmac: cleanup netdev transmit callback

The header of the ethernet packet is processed conditionally, but
the check is wrong as it checks skb length is at least ETH_ALEN. It
should check it is at least sizeof struct ethhdr instead.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
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>