xhci: replace USB_MAXINTERFACES with config->desc.bNumInterface

This patch replaces USB_MAXINTERFACES with config->desc.bNumInterface in
the termination condition for the loop that updates the LPM timeout of the
endpoints on the cofiguration's interfaces, in xhci_calculate_lpm_timeout(),
to avoid unnecessary loop cycles since most configurations come with 1-2
interfaces while USB_MAXINTERFACES is 32.

Signed-off-by: Xenia Ragiadakou <burzalodowa@gmail.com>
Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>