mux: asf: header: set correct number of sub objects

The Microsoft ASF spec states

Number of Header Objects
Specifies the number of objects contained within the Header Object, not
including this one. In addition, the Header Extension Object (sections 3.4
and 4) is counted as exactly one object regardless of the number of
subobjects contained within it.

This means i_cm_size should not contribute to the number of sub objects. As a result VLC generated ASF files could not be demuxed by gstreamer's asfdemux.

Fix the number and while at it give some semantic meaning to the magic numbers in the code regarding the header sub object.

Closes #28714 (closed)

Signed-off-by: Sebastian Gross sgross@emlix.com