greybus: fix manifest parsing size bug

The type-specific "create" routines that get called while parsing
the descriptor entries in the module manifest assume the size they
are provided is the size of their data portion only--not including
the descriptor header.

Compute this value in greybus_new_module(), and pass it to those
functions rather than the full descriptor size.  Move a few
declarations to the innermost block that uses them.

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