OneNAND: Add missing mtd info struct before calling onenand_erase()

Without this patch "saveenv" crashes when MTD partitions are enabled (e.g.
for use in UBI) via CONFIG_MTD_PARTITIONS.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>
diff --git a/common/env_onenand.c b/common/env_onenand.c
index 3c65b3e..dbccc79 100644
--- a/common/env_onenand.c
+++ b/common/env_onenand.c
@@ -97,6 +97,7 @@
 
 	instr.len = CONFIG_ENV_SIZE;
 	instr.addr = env_addr;
+	instr.mtd = &onenand_mtd;
 	if (onenand_erase(&onenand_mtd, &instr)) {
 		printf("OneNAND: erase failed at 0x%08lx\n", env_addr);
 		return 1;