mtd: spi-nor-core: Fix index value for SCCR dwords

Array index for SCCR 22th DWORD should be 21.

Fixes: bebdc237 ("mtd: spi-nor: Parse SFDP SCCR Map")
Signed-off-by: Takahiro Kuwano <Takahiro.Kuwano@infineon.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>