[SERIAL] serial_cs: convert IBM post-init handling to a quirk

Move IBM quirk handling into its own quirk entry.  Note that doing
quirk handling after we've registered the ports is racy, but since
I don't know if moving this will have an undesired effect, it's
probably better to leave where it is.

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