powerpc: Fix build failure when CONFIG_USB=y

We are enabling USB unconditionally which results in following build failure

drivers/built-in.o: In function `tb_drom_read':
(.text+0x1b62b70): undefined reference to `usb_speed_string'
make: *** [vmlinux] Error

Enable USB only if USB_SUPPORT is set to avoid such failures
Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Acked-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/44x/Kconfig

@@ -215,7 +215,7 @@ config AKEBONO
 	select NET_VENDOR_IBM
 	select IBM_EMAC_EMAC4
 	select IBM_EMAC_RGMII_WOL
-	select USB
+	select USB if USB_SUPPORT
 	select USB_OHCI_HCD_PLATFORM if USB_OHCI_HCD
 	select USB_EHCI_HCD_PLATFORM if USB_EHCI_HCD
 	select MMC_SDHCI