USB: xhci: Refactor code to free or cache endpoint rings.

Refactor out the code to cache or free endpoint rings from recently
dropped or disabled endpoints.  This code will be used by a new function
to reset a device and disable all endpoints except control endpoint 0.

Signed-off-by: Sarah Sharp <sarah.a.sharp@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>