serial: mcf: fix no driver name in probe output

The current output of the device probing looks like this:

ColdFire internal UART serial driver
(NULL device *): ttyS0 at MMIO 0xfc060000 (irq = 90, base_baud = 5208333) is a ColdFire UART
console [ttyS0] enabled
(NULL device *): ttyS1 at MMIO 0xfc064000 (irq = 91, base_baud = 5208333) is a ColdFire UART
(NULL device *): ttyS2 at MMIO 0xfc068000 (irq = 92, base_baud = 5208333) is a ColdFire UART

Fix the port device linkage at probing time so that the device name is
printed out properly, and not "(NULL device *)".

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>