xhci: add variable 'cmd_type' in handle_cmd_completion()

This patch adds a new variable 'cmd_type' to hold the command type so that
switch cases can be simplified by removing TRB_TYPE() macro improving
code readability.

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