USB: legotower: 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).

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>