mtd: cfi_cmdset_0002: fix SEGV unlocking multiple chips

cfi_ppb_unlock() tries to relock all sectors that were locked before
unlocking the whole chip.
This locking used the chip start address + the FULL offset from the
first flash chip, thereby forming an illegal address. Fix that by using
the chip offset(adr).

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>