powerpc/configs/6xx: Switch CONFIG_USB_EHCI_FSL to =m

In commit ca07e1c1 ("drivers:usb:fsl:Make fsl ehci drv an
independent driver module"), CONFIG_USB_EHCI_FSL was switched from
built-in to modular. Update the defconfig.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/configs/ppc6xx_defconfig

@@ -892,7 +892,7 @@ CONFIG_USB=y
 CONFIG_USB_ANNOUNCE_NEW_DEVICES=y
 CONFIG_USB_MON=y
 CONFIG_USB_EHCI_HCD=m
-CONFIG_USB_EHCI_FSL=y
+CONFIG_USB_EHCI_FSL=m
 CONFIG_USB_OHCI_HCD=m
 CONFIG_USB_OHCI_HCD_PPC_OF_BE=y
 CONFIG_USB_OHCI_HCD_PPC_OF_LE=y