dynamic: Prevent oops due to inverted compile flag

The logic on ENABLE_TASKELETS compiler flag was inverted causing an oops on
normal dahdi_cfg of a dynamic span.

Issue-id: https://issues.asterisk.org/jira/browse/DAHLIN-328

Signed-off-by: Russ Meyerriecks <rmeyerriecks@digium.com>
Acked-by: Shaun Ruffell <sruffell@digium.com>