Bluetooth: Prepare start_discovery

This patch does some code refactoring in start_discovery function
in order to prepare it for interleaved discovery support.

MGMT_ADDR_* macros were moved to hci_core.h since they are now used
to define discovery type macros.

Discovery type macros were defined according to mgmt-api.txt
specification:

Possible values for the Type parameter are a bit-wise or of the
following bits:

	1	BR/EDR
	2	LE Public
	3	LE Random

By combining these e.g. the following values are possible:

	1	BR/EDR
	6	LE (public & random)
	7	BR/EDR/LE (interleaved discovery)

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>