crypto: ccp - Rework the unit-size check for XTS-AES

The CCP supports a limited set of unit-size values. Change the check
for this parameter such that acceptable values match the enumeration.
Then clarify the conditions under which we must use the fallback
implementation.

Signed-off-by: Gary R Hook <gary.hook@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>