aspeed/smc: Calculate checksum on normal DMA

This patch adds the missing checksum calculation on normal DMA transfer.
According to the datasheet this is how the SMC should behave.

Verified on AST1250 that the hardware matches the behaviour.

Signed-off-by: Christian Svensson <bluecmd@google.com>
Reviewed-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20190904070506.1052-9-clg@kaod.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>