serial: mvebu-uart: fix unused variable warning

There's a warning shows that 'ret' becomes an unused variable
after simplify the return expression of mvebu_uart_probe(). So
remove it.

Fixes: b6353702

 ("serial: mvebu-uart: simplify the return expression of mvebu_uart_probe()")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
Link: https://lore.kernel.org/r/20200929085651.158283-1-miaoqinglang@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>