Bluetooth: Fix HCI command send functions to use const specifier

All HCI command send functions that take a pointer to the command
parameters do not need to modify the content in any way (they merely
copy the data to an skb). Therefore, the parameter type should be
declared const. This also allows passing already const parameters to
these APIs which previously would have generated a compiler warning.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>