qcacmn: Add pdev param to set firmware to use lower chains for tx

Add new pdev param with which firmware can be set to use
a lower number of chains without doing a reset. In response
to this firmware's ratecontrol will select only rates
which can be sent using a lower number of chains, and set
the h/w tx descpriptor's chain select to whatever value
has been configured using this command.

CRs-Fixed: 2123206
Change-Id: Ice713393958c2c2acedba0fe2e9354f3fe3bee75

wmi_unified_param.h

@@ -5550,6 +5550,7 @@ typedef enum {
 	wmi_pdev_param_btcoex_cfg,
 	wmi_pdev_param_mesh_mcast_enable,
 	wmi_pdev_param_tx_ack_timeout,
+	wmi_pdev_param_soft_tx_chain_mask,
 
 	wmi_pdev_param_max,
 } wmi_conv_pdev_params_id;