spl: Convert spl_usb_load_image() to use linker list

Add a linker list declaration for this method and remove the explicit
switch() code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/common/spl/spl.c b/common/spl/spl.c
index 84ae003..d2e23bf 100644
--- a/common/spl/spl.c
+++ b/common/spl/spl.c
@@ -400,9 +400,6 @@
 		bootdev.boot_device_name = "usb_ether";
 		return spl_net_load_image(&bootdev);
 #endif
-#ifdef CONFIG_SPL_USB_SUPPORT
-	case BOOT_DEVICE_USB:
-		return spl_usb_load_image(&bootdev);
 #endif
 #ifdef CONFIG_SPL_SATA_SUPPORT
 	case BOOT_DEVICE_SATA:
diff --git a/common/spl/spl_usb.c b/common/spl/spl_usb.c
index f990336..2bc321a 100644
--- a/common/spl/spl_usb.c
+++ b/common/spl/spl_usb.c
@@ -22,7 +22,7 @@
 static int usb_stor_curr_dev = -1; /* current device */
 #endif
 
-int spl_usb_load_image(struct spl_boot_device *bootdev)
+static int spl_usb_load_image(struct spl_boot_device *bootdev)
 {
 	int err;
 	struct blk_desc *stor_dev;
@@ -61,3 +61,4 @@
 
 	return 0;
 }
+SPL_LOAD_IMAGE_METHOD(0, BOOT_DEVICE_USB, spl_usb_load_image);
diff --git a/include/spl.h b/include/spl.h
index 7885521..7270cc2 100644
--- a/include/spl.h
+++ b/include/spl.h
@@ -187,9 +187,6 @@
 /* Ethernet SPL functions */
 int spl_net_load_image(struct spl_boot_device *bootdev);
 
-/* USB SPL functions */
-int spl_usb_load_image(struct spl_boot_device *bootdev);
-
 /* SATA SPL functions */
 int spl_sata_load_image(struct spl_boot_device *bootdev);