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>

common/env_onenand.c

@@ -97,6 +97,7 @@ int saveenv(void)
 
 	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;