USB: s3c2410_udc: minor irq handler cleanups

- 'irq' argument is merely used in place of a constant; replace its usage
  with that constant.

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Cc: David Brownell <david-b@pacbell.net>
Cc: Greg KH <greg@kroah.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>