USB: s3c-hsotg: Avoid overwriting contents of perodic in 'fifo'

In shared fifo mode (used on older SoCs) the periodic in fifo beahves
much more like a packet buffer, discarding old data when writing new
data. Avoid this by ensuring that we do not load new transactions in
when there is data sitting already in the FIFO.

Note, this may not be an observed bug, we are fixing the case that this
may happen.

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