Bluetooth: Query LE tx power on startup

Queries tx power via HCI_LE_Read_Transmit_Power command when the hci
device is initialized, and stores resulting min/max LE power in hdev
struct. If command isn't available (< BT5 support), min/max values
both default to HCI_TX_POWER_INVALID.

This patch is manually verified by ensuring BT5 devices correctly query
and receive controller tx power range.

Reviewed-by: Sonny Sasaka <sonnysasaka@chromium.org>
Signed-off-by: Daniel Winkler <danielwinkler@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>