staging: wfx: better naming for hif_mib_set_association_mode->greenfield

Current name "mixed_or_greenfield_type" does not allow to know if
"true" means "mixed" of "greenfield". It is possible to use a better
name and drop "enum hif_tx_mode".

Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191217161318.31402-29-Jerome.Pouiller@silabs.com


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