crypto: ccp - Cleanup misc_dev on sev_exit()

Explicitly free and clear misc_dev in sev_exit(). Since devm_kzalloc()
associates misc_dev with the first device that gets probed, change from
devm_kzalloc() to kzalloc() and explicitly free memory in sev_exit() as
the first device probed is not guaranteed to be the last device released.
To ensure that the variable gets properly set to NULL, remove the local
definition of misc_dev.

Fixes: 200664d5 ("crypto: ccp: Add Secure Encrypted Virtualization (SEV) command support")
Signed-off-by: John Allen <john.allen@amd.com>
Acked-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>