USB: musb_host, fix ep0 fifo flushing

The MUSB host side can't share generic TX FIFO flush logic
with EP0; the EP0 TX status register bits are different
from those for other entpoints.

Resolve this issue by providing a new EP0-specific routine
to flush and reset the FIFO, which pays careful attention to
restrictions listed in the latest programmer's guide.  This
gets rid of an open issue whereby the usbtest control write
test (#14) failed.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>