mx8m: csf.sh: Fix the calculation of fit_block_size

When running the script to sign SPL/U-Boot on a kontron-sl-mx8mm board,
the fit_block_size was calculated as 0x1000 instead of 0x1020.

Add an extra parenthesis pair to fix it.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>