xhci: Remove duplicate xhci urb giveback functions

We want one function that gives back urb in interrupt that can
be used in both normal cases and when killing off urbs.

Add small helpers that check if td is last_td_in_urb() and
that increase the td count of a urb inc_td_cnt()

Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>