spi: aspeed: Fix window offset of CE1

The offset value of the mapping window in the kernel structure is
calculated using the value of the previous window offset. This doesn't
reflect how the HW is configured and can lead to erroneous setting of
the second flash device (CE1).

Cc: Chin-Ting Kuo <chin-ting_kuo@aspeedtech.com>
Fixes: e3228ed9

 ("spi: spi-mem: Convert Aspeed SMC driver to spi-mem")
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Link: https://lore.kernel.org/r/20221016155722.3520802-1-clg@kaod.org
Signed-off-by: Mark Brown <broonie@kernel.org>