USB: s3c-hsotg: Export usb_gadget_register_driver()

USB gadget controller drivers normally export their driver registration
function, allowing modular builds of the individual gadget drivers so
do so for s3c-hsotg, fixing builds.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>