usb/isp1760: Replace period calculation for INT packets with something readable

Replace the period calculation for INT packets with something readable. Seems
to fix a rare bug with quickly repeated insertion/removal of several USB
devices simultaneously (hub control INT packets).

Signed-off-by: Arvid Brodin <arvid.brodin@enea.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>