crypto: ccp - simplify sev_update_firmware()

crypto: ccp - simplify sev_update_firmware()

sev_do_cmd() has its own command buffer (sev->cmd_buf) with the correct
alignment, perms, etc. that it copies the command into, so prepending it to
the firmware data is unnecessary.

Switch sev_update_firmware() to using a stack allocated command in light of
this copy, and drop all of the resulting pointer math.

Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>