There is no need to set and validate the configuration when the camera
is started, since this was already made when the camera was opened.
The configuration only needs to be set again if the port format changes
due a SPA_PARAM_Format parameter.
SPA_MEMBER is misleading, all we're doing here is pointer+offset and a
type-casting the result. Rename to SPA_PTROFF which is more expressive (and
has the same number of characters so we don't need to re-indent).
