HCI: Simplify btu_hcif_hdl_command_status()

* Move 2nd layer switch statements into top layer
* Check if p_cmd is nullptr at the beginning of the function so
  that each case does not need to check it individually. p_cmd
  is never null.
* Skip parameter total length at beginning of the function

Test: make, unit test, use Bluetooth for music and calls
Change-Id: I23cf983a1ade44bcea0b1009860efdfaf3b8bca8