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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index d61aa94..e4bf757 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -15,7 +15,7 @@
 CONFIG_SPL_NET_VCI_STRING="AM335x U-Boot SPL"
 CONFIG_SPL_OS_BOOT=y
 CONFIG_SPL_USB_GADGET_SUPPORT=y
-CONFIG_SPL_USBETH_SUPPORT=y
+CONFIG_SPL_USB_ETHER=y
 # CONFIG_SPL_YMODEM_SUPPORT is not set
 CONFIG_FASTBOOT=y
 CONFIG_CMD_SPL=y