Bluetooth: Convert hci_conn->link_mode into flags

Since the link_mode member of the hci_conn struct is a bit field and we
already have a flags member as well it makes sense to merge these two
together. This patch moves all used link_mode bits into corresponding
flags. To keep backwards compatibility with user space we still need to
provide a get_link_mode() helper function for the ioctl's that expect a
link_mode style value.

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