mtd: spi: Convert is_locked callback to is_unlocked

There was no user of this callback after 5b66fdb2 anymore, and its
semantic as now inconsistent between stm and sst26. What we need for the
upcoming new usecase is a "completely unlocked" semantic. So consolidate
over this.

Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>