crypto: ccp - Make function sev_get_firmware() static

Fixes the following sparse warning:

drivers/crypto/ccp/psp-dev.c:444:5: warning:
 symbol 'sev_get_firmware' was not declared. Should it be static?

Fixes: e9372060 ("crypto: ccp - Allow SEV firmware to be chosen based on Family and Model")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>