fwu: mtd: remove unused argument from function call

The third argument passed to the function gen_image_alt_info() is not
used and is superfluous. Remove this unused argument from the function
call.

Fixes: 4898679e (FWU: Add FWU metadata access driver for MTD storage regions)
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>