xhci: refactor TRB_DISABLE_SLOT case into function

The function that handles xHCI command completion is much too long and
there is need to be broken up into individual functions for each command
completion to improve code readablity.
This patch refactors the code in TRB_DISABLE_SLOT switch case in
handle_cmd_completion() into a fuction named xhci_handle_cmd_disable_slot().

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