serial: etraxfs-uart: Fix release etraxfs_uart_ports

In probe, we use dev_id as array index of etraxfs_uart_ports and store the
index in port->line. So etraxfs_uart_ports[port->line] should be released
when unload the module.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Niklas Cassel <nks@flawful.org>
Acked-by: Jesper Nilsson <jesper.nilsson@axis.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>