ath10k: add support for configuring management packet rate

By default the firmware uses 1Mbps and 6Mbps rate for management packets
in 2G and 5G bands respectively. But when the user selects different
basic rates from the userspace, we need to send the management
packets at the lowest basic rate selected by the user.

This change makes use of WMI_VDEV_PARAM_MGMT_RATE param for configuring the
management packets rate to the firmware.

Chipsets Tested : QCA988X, QCA9887, QCA9984
FW Tested 	: 10.2.4-1.0-41, 10.4-3.6.104

Signed-off-by: Sriram R <srirrama@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>