mtd: Use get_mtdids() instead of env_get("mtdids") in mtd_search_alternate_name()

The environment is not guaranteed to contain a valid mtdids variable
when called from mtd_search_alternate_name(). Call get_mtdids() instead
of env_get("mtdids").

Fixes: ff4afa8a

 ("mtd: uboot: search for an equivalent MTD name with the mtdids")
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>
Tested-by: Heiko Schocher <hs@denx.de>