staging: wfx: fix number of available tx_policies

Original API declares 16 tx_policies. But in fact, the 16th is used
internally by the firmware. So, only 15 tx_policies are available for
driver.

Reported-by: Alban Jeantheau <alban.jeantheau@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-6-Jerome.Pouiller@silabs.com


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