USB: s3c-hsotg: Ensure FIFOs are fully flushed after layout

According to the design guide, if the FIFO layout is changed, then the
FIFOs must be flushed to ensure all FIFO pointers are correct.

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>