usb: Sparc build fix, make USB_ISP1760_OF depend on PPC_OF

Sparc doesn't have some of the OF interfaces this driver
wants to use.

Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/usb/host/Kconfig

@@ -129,7 +129,7 @@ config USB_ISP1760_PCI
 
 config USB_ISP1760_OF
 	bool "Support for the OF platform bus"
-	depends on USB_ISP1760_HCD && OF
+	depends on USB_ISP1760_HCD && PPC_OF
 	---help---
 	  Enables support for the device present on the PowerPC
 	  OpenFirmware platform bus.