serial: altera_uart: Scan for a free port if platform device id is -1

Devices extracted from device tree all seem to have pdev->id set to -1.
Up until now we mapped all devices with id -1 to the first device.  This
behaviour could lead to problems when using more than one Altera UART in
a system.

This patch changes the behaviour of the driver to scan for the next free
id in case the id is -1.

Because we cannot refer back to the assigned id in altera_uart_remove,
the port instance needs to be stored in device drvdata.

Reported-by: David Smoot <davidsmoot@gmail.com>
Cc: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>