Bluetooth: hci_intel: Add Intel baudrate configuration support

Implement the set_baudrate callback for hci_intel.
 - Controller requires a read Intel version command before updating
   its baudrate.
 - The operation consists in an async cmd since the controller does
   not respond at the same speed.
 - Wait 100ms to let the controller change its baudrate.
 - Clear RTS until we change our own UART speed

Manage speed change in the setup function, we need to restore the oper
speed once chip has booted on patched firmware.

Signed-off-by: Loic Poulain <loic.poulain@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>