rtl8150: use default MTU of 1500

The RTL8150 driver uses an MTU of 1540 by default, which causes a
bunch of problems -- it prevents booting from NFS root, for one.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>
Cc: Petko Manolov <petkan@nucleusys.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: David S. Miller <davem@davemloft.net>