greybus: manifest: Account for padding in string descriptor

String descriptors are padded towards the end to align them to 4 byte
boundaries. Take that into account while calculating expected size.

Reviewed-by: Alex Elder <elder@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>