wifi: wfx: prevent underflow in wfx_send_pds()

This does a "chunk_len - 4" subtraction later when it calls:

	ret = wfx_hif_configuration(wdev, buf + 4, chunk_len - 4);

so check for "chunk_len" is less than 4.

Fixes: dcbecb49 ("staging: wfx: allow new PDS format")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/Yv8eX7Xv2ubUOvW7@kili