xhci: rename existing Command Completion Event handlers

This patch renames the function handlers of a triggered Command Completion
Event that correspond to each command type into 'xhci_handle_cmd_<type>'.
That is done to give a consistent naming space to all the functions that
handle Command Completion Events and that will permit the code reader to
reference to them more easily.

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