usb: host: xhci: use __ffs() instead of hardcoding shift

__ffs() can tell us which is the SEGMENT_SHIFT value
to be used. This will prevent problems when users are
too fast and don't pay attention to the need of fixing
the Shift after changing TRBS_PER_SEGMENT.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>