mtd: cfi_cmdset_0002, fix lock imbalance

Stanse found a double unlock in get_chip. get_chip is called with
chip->mutex held and caller is responsible for unlocking it too.

Do not unlock the lock in get_chip on a fail path. This would mean
a double unlock.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>