xhci: add variable 'cmd_comp_code' in handle_cmd_completion()

This patch adds a new variable 'cmd_comp_code' to hold the command completion
status code aiming to reduce code duplication and to improve code readability.

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>