USB: s3c-hsotg: Only load packet per fifo write

Limit the IN FIFO write to a single packet per attempt at writing,
as per the specifications and ensure that we don't return fifo-full
so that we can continue writing packets if we have the space.

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