net: fec: init maximum receive buffer size for ring1 and ring2

i.MX6SX fec support three rx ring1, the current driver lost to init
ring1 and ring2 maximum receive buffer size, that cause receving
frame date length error. The driver reports "rcv is not +last" error
log in user case.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>