serial: mvebu-uart: simplify the return expression of mvebu_uart_probe()

Simplify the return expression.

Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Link: https://lore.kernel.org/r/20200921131105.93177-1-miaoqinglang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>