net: sparx5: fix the maximum frame length register

On port initialization, we configure the maximum frame length accepted
by the receive module associated with the port. This value is currently
written to the MAX_LEN field of the DEV10G_MAC_ENA_CFG register, when in
fact, it should be written to the DEV10G_MAC_MAXLEN_CFG register. Fix
this.

Fixes: 946e7fd5 ("net: sparx5: add port module support")
Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>