Bluetooth: Remove unneeded mgmt_discoverable function

Since the HCISETSCAN ioctl is the only non-mgmt user we care about for
setting the right discoverable state we can simply do the necessary
updates in the ioctl handler function instead. This then allows the
removal of the mgmt_discoverable function and should simplify that state
handling considerably.

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