At the beginning of the function we gain a reference to the driver
capabilities. Then, we call format function (*) which if failed, unref
over caps is called. Then, at the end another unref occurs.

* - Moreover, the format was not called over gained caps, but over
privconn->caps directly which is not allowed anymore.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>