Check cipher key lengths during probesession.

OCF drivers in general should perform as many session parameter checks
as possible during probesession rather than when creating a new
session. I got this wrong for aesni(4) in r359374. In addition,
aesni(4) was performing the check for digest-only requests and failing
to create digest-only sessions as a result.

Reported by: jkim
Tested by: jkim
Sponsored by: Chelsio Communications