mtd: cfi_cmdset_0002: dynamically determine the max sectors

Because PPB unlocking unlocks the whole chip cfi_ppb_unlock() needs to
remember the locked status for each sector so it can re-lock the
unaddressed sectors. Dynamically calculate the maximum number of sectors
rather than using a hardcoded value that is too small for larger chips.

Tested with Spansion S29GL01GS11TFI flash device.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Stefan Roese <sr@denx.de>
Acked-by: Vignesh Raghavendra <vigneshr@ti.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>