bcm63xx_enet: add xmit_more support

Support bulking hardware TX queue by using netdev_xmit_more().

Signed-off-by: Sieng Piaw Liew <liew.s.piaw@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>