USB: Move usb_disabled() towards top of the file

Move usb_disabled() and module_param()/core_param() towards the top of the file,
where 'nousb' is defined, as they are all related.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>