greybus: firmware: add comment to show size calculations

The firmware file name's maximum length is set to 56 right now, but it
isn't very much readable how we got to that value (unless someone reads
that line).

Update the comment to show calculations.

Reported-by: Alex Elder <alex.elder@linaro.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>