crypto: virtio - don't use 'default m'

Drivers shouldn't be enabled by default unless there is a very good
reason to do so.  There doesn't seem to be any such reason for the
virtio crypto driver, so change it to the default of 'n'.

Signed-off-by: Ram Muthiah <rammuthiah@google.com>
[EB: adjusted commit message]
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>