isa: Clean up inappropriate hw_error()

isa_bus_irqs(), isa_create() and isa_try_create() call hw_error() when
passed a null bus.  Use of hw_error() has always been questionable,
because these are used only during machine initialization, and
printing CPU registers isn't useful there.

Since the previous commit, passing a null bus is a programming error.
Drop the hw_error() and simply let it crash.

Cc: Richard Henderson <rth@twiddle.net>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Hervé Poussineau" <hpoussin@reactos.org>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Markus Armbruster <armbru@pond.sub.org>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
Message-Id: <1450354795-31608-12-git-send-email-armbru@redhat.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>
