serial: uartps: Move alias reading higher in probe()

This cosmetic change is done only for having next patch much easier to
read. Moving id setup higher in probe is not affecting any usage of this
driver and it also simplify error path.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>