xhci: refactor TRB_NEC_GET_FW 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_NEC_GET_FW switch case in
handle_cmd_completion() into a fuction named xhci_handle_cmd_nec_get_fw().

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