staging: wfx: improve protection against malformed HIF messages

As discussed here[1], if a message was smaller than the size of the
message header, it could be incorrectly processed.

[1] https://lore.kernel.org/driverdev-devel/2302785.6C7ODC2LYm@pc-42/



Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200701150707.222985-7-Jerome.Pouiller@silabs.com


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