Bluetooth: Fix default values for advertising interval

The DISCOV_LE_FAST_ADV_INT_{MIN,MAX} contants are in msec, but then used
later on directly while it is suppose to be N * 0.625 ms according to
the Bluetooth Core specification.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>