Add sub-command QCA_NL80211_VENDOR_SUBCMD_PEER_FLUSH_PENDING to flush
pending packets in firmware. The attributes are listed in enum
qca_wlan_vendor_attr_flush_pending. The QCA_WLAN_VENDOR_ATTR_PEER_ADDR
specifies the peer MAC address and the QCA_WLAN_VENDOR_ATTR_AC specifies
the access category of the pending packets.
Signed-off-by: Lin Tingting <tinlin@qti.qualcomm.com>