serial: vt8500: add missing braces

Due to missing braces on an if statement, in presence of a device_node a
port was always assigned -1, regardless of any alias entries in the
device tree. Conversely, if device_node was NULL, an unitialized port
ended up being used.

This patch adds the missing braces, fixing the issues.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Tony Prisk <linux@prisktech.co.nz>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>