tools: zynqmp_psu_init_minimize.sh: fix return lines coding style

Remove unneeded parenthess around return value. E.g.:
    return (0);   ->   return 0;

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>