net: macb: Remove obsolete comment from Kconfig

The Kconfig file says that Gigabit mode is not supported, but it has been
supported since commit 140b7552 ("net/macb:
Add support for Gigabit Ethernet mode").

Signed-off-by: James Byrne <james.byrne@origamienergy.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>