watchdog: xilinx: Move control_status_reg to functions

control_status_reg is temp variables and should be
used locally by specific function.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>