usb: isp1760: use msecs_to_jiffies for time conversion

This is only an API consolidation and should make things more readable
it replaces var * HZ / 1000 by msecs_to_jiffies(var).

Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>