xhci: Refactor command watchdog and fix split string.

In preparation for fixing this function for streams endpoints, refactor
code in the command watchdog timeout function into two new functions.
One kills all URBs on a ring (either stream or endpoint), the other
kills all URBs associated with an endpoint.  Fix a split string while
we're at it.

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