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/configs/mx6memcal_defconfig b/configs/mx6memcal_defconfig
index bf0b4f2..fdcef6b 100644
--- a/configs/mx6memcal_defconfig
+++ b/configs/mx6memcal_defconfig
@@ -12,7 +12,7 @@
 CONFIG_SPL=y
 CONFIG_SPL_USB_HOST_SUPPORT=y
 CONFIG_SPL_USB_GADGET_SUPPORT=y
-CONFIG_SPL_USBETH_SUPPORT=y
+CONFIG_SPL_USB_ETHER=y
 CONFIG_SPL_USB_SDP_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 # CONFIG_CMD_BOOTD is not set