crypto: ccp - Use lower 8 bytes to communicate with doorbell command register

Unlike other command registers used by the PSP, only the lower 8 bytes are
used for communication for both command and status of the command.

Suggested-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Reviewed-by: Mark Hasemeyer <markhas@chromium.org>
Tested-by: Mark Hasemeyer <markhas@chromium.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>