qcacld-3.0: Fix -Wmissing-prototypes in wlan_hdd_softap_tx_rx

We want to enable the compiler's -Wmissing-prototypes switch, but
there is existing code that is generating warnings. Fix all warnings
in wlan_hdd_softap_tx_rx.

Change-Id: I26dd53307454d2c4dd887345eb77a9d362b5fdf2
CRs-Fixed: 1074336

core/hdd/src/wlan_hdd_softap_tx_rx.c

@@ -211,7 +211,8 @@ void hdd_softap_tx_resume_cb(void *adapter_context, bool tx_resume)
  *
  * Return: Always returns NETDEV_TX_OK
  */
-int __hdd_softap_hard_start_xmit(struct sk_buff *skb, struct net_device *dev)
+static int __hdd_softap_hard_start_xmit(struct sk_buff *skb,
+					struct net_device *dev)
 {
 	sme_ac_enum_type ac = SME_AC_BE;
 	hdd_adapter_t *pAdapter = (hdd_adapter_t *) netdev_priv(dev);