net: mcf: check receive buffer size register value

ColdFire Fast Ethernet Controller uses a receive buffer size
register(EMRBR) to hold maximum size of all receive buffers.
It is set by a user before any operation. If it was set to be
zero, ColdFire emulator would go into an infinite loop while
receiving data in mcf_fec_receive. Add check to avoid it.

Reported-by: Wjjzhang <wjjzhang@tencent.com>
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>
