serial: Factor out common serial_hds_isa_init()

It's the same old loop copied five times, plus another instance where
it's clipped to two iterations and unrolled.

No external users of serial_isa_init() are left, so give it internal
linkage.

Maintainers of affected machines cc'ed.

Cc: Richard Henderson <rth@twiddle.net>
Cc: Anthony Liguori <aliguori@amazon.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
