rockchip: boot_mode: fix fastboot command

The USB controller index must be separated from the type argument,
otherwise the preboot command fails with the error:

	Error: Wrong USB controller index format

Add the missing space to fix fastboot mode here.

Signed-off-by: John Keeping <john@metanate.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/arch/arm/mach-rockchip/boot_mode.c b/arch/arm/mach-rockchip/boot_mode.c
index 2158934..1a1a887 100644
--- a/arch/arm/mach-rockchip/boot_mode.c
+++ b/arch/arm/mach-rockchip/boot_mode.c
@@ -95,7 +95,7 @@
 	switch (boot_mode) {
 	case BOOT_FASTBOOT:
 		debug("%s: enter fastboot!\n", __func__);
-		env_set("preboot", "setenv preboot; fastboot usb0");
+		env_set("preboot", "setenv preboot; fastboot usb 0");
 		break;
 	case BOOT_UMS:
 		debug("%s: enter UMS!\n", __func__);