serial/efm32-uart: don't use pdev->id to determine the port's line

pdev->id is not a valid choice for device-tree probed devices. So use
the (properly determined) line from efm32_uart_probe consistenly

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>