tools: mkfwumdata: fix the size parameter to the fwrite call

The fwrite call returns the number of bytes transferred as part of the
write only when the size parameter is 1. Pass the size parameter to
the library call as 1 so that the correct number of bytes transferred
are returned.

Fixes: fdd56bfd ("tools: Add mkfwumdata tool for FWU metadata image")
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>