Bluetooth: kconfig: Disable advertising extension until feature complete

Disable the controller advertising extension feature default setting
until the feature is complete. The zephyr host requires the
LE Advertising Set Terminated event to function.
Without this event a peripheral connection cannot pair because the
local on-air address is not set, and the advertising state will not be
cleaned up, so advertising cannot be started again.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>