mfd: Fixed gpio polarity of omap-usb gpio USB-phy reset

With commit 19403165 a main part of ehci-omap.c moved to
drivers/mfd/omap-usb-host.c created by commit 17cdd29d

.
Due to this reorganisation the polarity used to reset the
external USB phy changed and USB host doesn't recognize
any devices.

Signed-off-by: Juergen Kilb <J.Kilb@phytec.de>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>