afs: the assert seems to be inverted when using memcache

afs: the assert seems to be inverted when using memcache

needvalid will always be 1 for the memory cache -- i.e. the slots are
always valid.  see change id I53ea6e99649e4d6d5cbde58929dfcee1d45a3e7b.

Change-Id: Id1294d54cb5f20d226258e5da39fb4d4f782a694
Reviewed-on: http://gerrit.openafs.org/7421
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Alistair Ferguson <alistair.ferguson@mac.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>