omap3: beagle: Fix USB host port power control

The host port power is enabled by driving the nEN_USB_PWR low as stated in
the comment. This fix is originally from Steve Sakoman <steve@sakoman.com>.

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Cc: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>