drivers/fm: Fix compiling error if FW location is not defined

FMAN firmware can be in NOR flash, NAND flash, SPI flash, MMC or even
remote. In case none of them is defined, set it to null.

Signed-off-by: York Sun <yorksun@freescale.com>
diff --git a/drivers/net/fm/fm.c b/drivers/net/fm/fm.c
index 4bc8f35..bca20b3 100644
--- a/drivers/net/fm/fm.c
+++ b/drivers/net/fm/fm.c
@@ -396,6 +396,8 @@
 	}
 #elif defined(CONFIG_SYS_QE_FMAN_FW_IN_REMOTE)
 	void *addr = (void *)CONFIG_SYS_QE_FMAN_FW_ADDR;
+#else
+	void *addr = NULL;
 #endif
 
 	/* Upload the Fman microcode if it's present */