mips/bootm: Fix typo in commit c4f9419c, "initrd_start" replaced by "images->rd_start"

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
diff --git a/lib_mips/bootm.c b/lib_mips/bootm.c
index c1bf21e..dced28c 100644
--- a/lib_mips/bootm.c
+++ b/lib_mips/bootm.c
@@ -73,7 +73,7 @@
 	linux_env_set ("memsize", env_buf);
 
 	sprintf (env_buf, "0x%08X", (uint) UNCACHED_SDRAM (images->rd_start));
-	linux_env_set ("images->rd_start", env_buf);
+	linux_env_set ("initrd_start", env_buf);
 
 	sprintf (env_buf, "0x%X", (uint) (images->rd_end - images->rd_start));
 	linux_env_set ("initrd_size", env_buf);