[SERIAL] Fix serial8250 driver initialisation ordering

Commit 7493a314

 changed the ordering
of the registration of the platform device driver vs the 8250 drivers
internal initialisation.  This led to the probe function being called
before the driver had finished its internal initialisation, causing
mayhem.  Revert the ordering change.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>