ARM: OMAP3: Beagle: Fix USB Host on beagle xM Ax/Bx

On Beagle xM Rev. Ax/Bx, the USB power enable GPIO logic is
reversed when compared to other revisions i.e. it is
active high instead of active low.

Use the beagle_config.usb_pwr_level flag correctly so that
the power regulator can be configured at runtime.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>