powerpc: booke_wdt: Fix build error as a module

Building booke_wdt fails when trying to build as a module as there is no
early_param() in module. Fix by using module_param() instead of early_param().

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>