PATCH: some gspcav2 fixes

Hi Jean-Francois,

While preparing clean patches for the removal of some came specific format 
decoding I noticed that I have forgotten to submit one important bugfix.

With gspcav2-2.0.4 S_FMT will return the internal format number instead of 0 
(for success) when the format does not need changing.

This patch also adds SPCA501 and SPCA561 pixelformats as preperation for my 
next patches.

Regards,

Hans

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