xhci: rename ep_ring variable in queue_bulk_tx(), no functional change

Tiny change, a bit more readable.
The real reason for this change is that the coming td fragment work
had several over 80 lines character lines split just because of a few
extra characters in variable names.

no functional changes

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