[connman] service: cancel agent requests prior to deleting wispr context

Calling __connman_wispr_stop() without connman_agent_cancel() allows pending
wispr requests to complete later which results in a read/write access to the
freed memory and a subsequent crash.

Calling connman_agent_cancel() without __connman_wispr_stop() stops the wispr
sequence which renders the wispr context useless. That makes no sense either.
Merge pull request #89 from monich/wispr_crash

service: cancel agent requests prior to deleting wispr context