block2mtd: dubious assignment

When block2mtd_erase fails, a duplicated assignment instantly
changes instr->state from MTD_ERASE_FAILED to MTD_ERASE_DONE.

It looks to me like this might not be intended, or is it?

Signed-off-by: Nicolas Kaiser <nikai@nikai.net>
Acked-By: Joern Engel <joern@logfs.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>