USB: pxa27x_udc: introduce pxa27x_clear_otgph()

Follow pxa27x change in OTGPH handling, and use the newly
defined pxa27x_clear_otgph().

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Acked-by: Eric Miao <eric.miao@marvell.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>