greybus: manifest: use size_t for a size variable

In identify_descriptor(), the variable desc_size represents the size
of a memory object.  So change its type from int to size_t.

The return value for this function can be desc_size cast to int.
One can verify by inspection this will never exceed INT_MAX.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>