microblaze: Fix a typo when disabling stack protection

Correct a typo causing the stack protector to be left enabled.
0xFFFFFFF -> 0xFFFFFFFF

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>