staging: wfx: fix case where RTS threshold is 0

If RTS threshold is 0, it currently disables RTS. It should mean
"enabled for every frames".

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20200115135338.14374-44-Jerome.Pouiller@silabs.com


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