usb: tegra: Fix typo in tegra20-usb-phy documentation

The device tree binding documentation for tegra20-usb-phy used to claim
that the vbus-supply property is required for host phys and optional
for otg phys. This should be the other way around.
Signed-off-by: NMikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: NFelipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/nvidia,tegra20-usb-phy.txt

@@ -48,5 +48,5 @@ Optional properties:
       peripheral means it is device controller
       otg means it can operate as either ("on the go")
 
-VBUS control (required for dr_mode == host, optional for dr_mode == otg):
+VBUS control (required for dr_mode == otg, optional for dr_mode == host):
   - vbus-supply: regulator for VBUS