serial: uartps: Fix missing unlock on error in cdns_get_id()

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

Fixes: ae1cca3f ("serial: uartps: Change uart ID port allocation")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>