usb: gadget: s3c-hsudc: delete outdated comment

since commit d93e2600 (usb: gadget: s3c-hsudc: use udc_start and udc_stop
functions) the 'driver' parameter has been deleted from
s3c_hsudc_stop_activity() but its documentation was left outdated. This
patch deletes the comment since it makes no sense anymore.

Signed-off-by: Chen Gang <gang.chen@asianux.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>