tty: serial: omap_serial: line is unsigned, don't check < 0

Dan Carpenter reported:
|drivers/tty/serial/8250/8250_omap.c:1025 omap8250_probe()
|warn: unsigned 'up.port.line' is never less than zero.
|1025          if (up.port.line < 0) {

Since of_alias_get_id() and pdev->id can get negative I check for the
error via ret variable.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>