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: NAlban Jeantheau <alban.jeantheau@silabs.com>
Signed-off-by: NJérôme Pouiller <jerome.pouiller@silabs.com>
Link: https://lore.kernel.org/r/20191017093954.657-6-Jerome.Pouiller@silabs.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wfx/hif_api_mib.h

@@ -507,7 +507,7 @@ struct hif_mib_tx_rate_retry_policy {
 	uint8_t    rates[12];
 } __packed;
 
-#define HIF_MIB_NUM_TX_RATE_RETRY_POLICIES    16
+#define HIF_MIB_NUM_TX_RATE_RETRY_POLICIES    15
 
 struct hif_mib_set_tx_rate_retry_policy {
 	uint8_t    num_tx_rate_policies;