mtd: remove duplicate assignment of chip->state

This is a trivial patch which removes unnecessary assignment of chip->state
in put_chip(). It's duplicated.

Signed-off-by: Tadashi Abe <tabe@mvista.com>
Acked-by: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>