greybus: manifest: simplify descriptor address calculation

This patch doesn't change any functionality. It just improves the
readability of the code.
Current code to get 'descriptors' pointer looks as if we are forcing the
pointer type change. To simplify the address calculations, use
'descriptors' member directly from greybus_manifest structure.

Signed-off-by: Sachin Pandhare <sachin.pandhare@linaro.org>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>