Bluetooth: Convert HCI_CONN_MASTER flag to a conn->role variable

Having a dedicated u8 role variable in the hci_conn struct greatly
simplifies tracking of the role, since this is the native way that it's
represented on the HCI level.

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