res_stasis_device_state: Set resource type for subscriptions to deviceState

The documentation for ARI already specifies that the device state resource when
used for subscribing for events is "deviceState", not "device_state". The code,
however, used "device_state"; although this was inconsistent as well in doxygen
comments in resource_applications.

Because the actual resource being subscribed to is /deviceStates/{device}/, it
makes sense for the resource type specifier to be deviceState.

Note that the key value in the events is still "device_state".
........

Merged revisions 404437 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@404438 65c4cc65-6c06-0410-ace0-fbb531ad65f3