staging: wfx: allow to send commands to chip

Chip has multiple input buffers and can handle multiple 802.11 frames
in parallel. However, other HIF command must be sent sequentially.
wsm_send_cmd() handles these requests.

This commit also add send_hif_cmd in debugfs. This file allows to send
arbitrary commands to chip. It can be used for debug and testing.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20190919142527.31797-12-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>