staging: wfx: better naming for hif_req_join->short_preamble

HIF_PREAMBLE_SHORT_LONG12 is never used. So it is possible to change
"preamble_type" into a boolean and drop "enum hif_preamble".

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-28-Jerome.Pouiller@silabs.com


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