xhci: fix integer overflow

xhci_service_interval_to_ns() returns long long
to avoid an overflow. However, the type cast happens
too late. The fix is to force ULL from the beginning.

This patch should be backported to kernels as old as 3.5, that contain
the commit e3567d2c "xhci: Add Intel
U1/U2 timeout policy."

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
CC: stable@vger.kernel.org