net: Update dahdi for alloc_netdev() api change in 3.17+

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=c835a677331495cf137a7f8a023463afd9f0~

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