staging: wfx: ensure that retry policy always fallbacks to MCS0 / 1Mbps

When not using HT mode, minstrel always includes 1Mbps as fallback rate.
But, when using HT mode, this fallback is not included. Yet, it seems
that it could save some frames. So, this patch add it unconditionally.

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-8-Jerome.Pouiller@silabs.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>