mtd: Initialize ->fail_addr early in mtd_erase()

mtd_erase() can return an error before ->fail_addr is initialized to
MTD_FAIL_ADDR_UNKNOWN. Move this initialization at the very beginning
of the function.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>