treewide: Test on CONFIG_USB_HOST (or CONFIG_CMD_USB) not CONFIG_USB

As the logic here is only used when we have a USB host controller, test
on CONFIG_USB_HOST rather than CONFIG_USB in general.  This lets us move
towards using CONFIG_USB only as a menu symbol to say that we have some
form of USB, and then USB_HOST or USB_GADGET depending on the role that
USB plays within the build.

Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/board/freescale/mpc837xerdb/mpc837xerdb.c b/board/freescale/mpc837xerdb/mpc837xerdb.c
index 66b3d9a..84671f6 100644
--- a/board/freescale/mpc837xerdb/mpc837xerdb.c
+++ b/board/freescale/mpc837xerdb/mpc837xerdb.c
@@ -220,7 +220,7 @@
 int board_late_init(void)
 {
 	volatile immap_t *immap = (immap_t *) CONFIG_SYS_IMMR;
-#ifdef CONFIG_USB
+#ifdef CONFIG_USB_HOST
 	clrsetbits_be32(&immap->sysconf.sicrl, SICRL_USB_A, 0x40000000);
 #endif
 	return 0;