usb: Don't enable CONFIG_SPL_DM_USB for non-DM SPL

When CONFIG_SPL_DM_USB was added, it was added defaulted on without
protection to ensure it didn't get set for non-DM SPL builds.

This leads to unexpected and confusing failures when building a DM based
U-Boot but with a non-DM SPL, as the defconfig is unlikely to have
CONFIG_SPL_DM_USB actively disabled.

Tweak the Kconfig to not enable this setting unless CONFIG_SPL_DM is set.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>
diff --git a/drivers/usb/Kconfig b/drivers/usb/Kconfig
index 98f8343..3b53bf2 100644
--- a/drivers/usb/Kconfig
+++ b/drivers/usb/Kconfig
@@ -49,7 +49,7 @@
 
 config SPL_DM_USB
 	bool "Enable driver model for USB in SPL"
-	depends on DM_USB
+	depends on SPL_DM && DM_USB
 	default y
 
 config DM_USB_GADGET