serial: 8250: 8250_core: Fix missing unlock on error in serial8250_register_8250_port()

Add the missing unlock before return from function serial8250_register_8250_port()
in the error handling case.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Link: https://lore.kernel.org/r/20190703055908.141294-1-weiyongjun1@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>