mtd: cfi_cmdset_0002: Avoid walking all chips when unlocking.

cfi_ppb_unlock() walks all flash chips when unlocking sectors,
avoid walking chips unaffected by the unlock operation.

Fixes: 1648eaaa

 ("mtd: cfi_cmdset_0002: Support Persistent Protection Bits (PPB) locking")
Cc: stable@vger.kernel.org
Signed-off-by: Joakim Tjernlund <joakim.tjernlund@infinera.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>