Bluetooth: Add support for Start Service Discovery command

This patch adds support for the Start Service Discovery command. It
does all the checks for command parameters and configured the discovery
filter settings correctly. However the actual support for filtering
will be added with another patch.

Signed-off-by: Jakub Pawlowski <jpawlowski@google.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>