serial: 8250_uniphier: fix error return code in uniphier_uart_probe()

Fix to return a negative error code from the port register error
handling case instead of 0, as done elsewhere in this function.

Fixes: 39be40ce ("serial: 8250_uniphier: fix serial port index in private data")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>