spl: Kconfig: Rename SPL_USBETH_SUPPORT to SPL_USB_ETHER to match with the U-boot CONFIG

Rename CONFIG_SPL_USBETH_SUPPORT to CONFIG_SPL_USB_ETHER.

This enables users to block text using CONFIG_IS_ENABLED() instead
of resorting to #if ladders with SPL and non-SPL cases.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
diff --git a/include/configs/am335x_sl50.h b/include/configs/am335x_sl50.h
index 569220e..739a998 100644
--- a/include/configs/am335x_sl50.h
+++ b/include/configs/am335x_sl50.h
@@ -74,7 +74,7 @@
 /* Bootcount using the RTC block */
 #define CONFIG_SYS_BOOTCOUNT_BE
 
-#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_USBETH_SUPPORT)
+#if defined(CONFIG_SPL_BUILD) && defined(CONFIG_SPL_USB_ETHER)
 /* Remove other SPL modes. */
 /* disable host part of MUSB in SPL */
 #undef CONFIG_MUSB_HOST