greybus: firmware: s/_LEN/_SIZE

Alex Elder pointed out that the macros also count the trailing NULL
('\0') character and so it should be using SIZE instead of LEN.

This patch makes that change.

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>