staging: wfx: send just necessary bytes

Size of hif_mib_template_frame must be sufficient to contains bytes
declared by frame_length and cannot exceed 700bytes.

Change the API to reflect that.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200406111756.154086-12-Jerome.Pouiller@silabs.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>