usb: CONFIG_USB_FASTBOOT prefix replacement for consistency

FASTBOOT is defined both by CONFIG_USB_FUNCTION_FASTBOOT AND CONFIG_CMD_FASTBOOT, so it doesn't
make much sense to have a CONFIG_USB_FASTBOOT prefix for fastboot-specific options, especially
given that other config options for fastboot use the CONFIG_FASTBOOT prefix.

This replaces the CONFIG_USB_FASTBOOT prefix with CONFIG_FASTBOOT, for consistency.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
diff --git a/include/configs/nitrogen6x.h b/include/configs/nitrogen6x.h
index 47239b1..3bb2e8f 100644
--- a/include/configs/nitrogen6x.h
+++ b/include/configs/nitrogen6x.h
@@ -349,7 +349,7 @@
 #define CONFIG_USB_FUNCTION_FASTBOOT
 #define CONFIG_CMD_FASTBOOT
 #define CONFIG_ANDROID_BOOT_IMAGE
-#define CONFIG_USB_FASTBOOT_BUF_ADDR   CONFIG_SYS_LOAD_ADDR
-#define CONFIG_USB_FASTBOOT_BUF_SIZE   0x07000000
+#define CONFIG_FASTBOOT_BUF_ADDR   CONFIG_SYS_LOAD_ADDR
+#define CONFIG_FASTBOOT_BUF_SIZE   0x07000000
 
 #endif	       /* __CONFIG_H */