octeon_ep: Fix error code in probe()

Set the error code if octep_ctrl_net_get_mtu() fails.  Currently the code
returns success.

Fixes: 0a5f8534 ("octeon_ep: get max rx packet length from firmware")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Sathesh B Edara <sedara@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>