am33xx: Board: Make CPSW section of ethernet initialization depend on CPSW driver

Not doing so breaks cases where CPSW is not required such as for USB RNDIS network boot.
Signed-off-by: NJoel A Fernandes <joelagnel@ti.com>

board/ti/am335x/board.c

@@ -505,6 +505,7 @@ int board_eth_init(bd_t *bis)
 			eth_setenv_enetaddr("ethaddr", mac_addr);
 	}
 
+#ifdef CONFIG_DRIVER_TI_CPSW
 	if (board_is_bone() || board_is_bone_lt() || board_is_idk()) {
 		writel(MII_MODE_ENABLE, &cdev->miisel);
 		cpsw_slaves[0].phy_if = cpsw_slaves[1].phy_if =
@@ -520,6 +521,7 @@ int board_eth_init(bd_t *bis)
 		printf("Error %d registering CPSW switch\n", rv);
 	else
 		n += rv;
+#endif
 
 	/*
 	 *