mtd: spi-nor-core: Invert logic to reflect sst26 flash unlocked

flash_is_locked is changed to flash_is_unlocked with commit 513c6071
("mtd: spi: Convert is_locked callback to is_unlocked"). sst26_is_locked()
is also changed to sst26_is_unlocked() but the logic remained same.
Invert the logic for the flash lock/unlock to work properly.

Signed-off-by: Algapally Santosh Sagar <santoshsagar.algapally@amd.com>
Signed-off-by: Ashok Reddy Soma <ashok.reddy.soma@amd.com>
Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Link: https://lore.kernel.org/r/20221122051833.13306-1-ashok.reddy.soma@amd.com


Signed-off-by: Michal Simek <michal.simek@amd.com>