crypto: aesni - Fix failure when pcbc module is absent

When aesni is built as a module together with pcbc, the pcbc module
must be present for aesni to load.  However, the pcbc module may not
be present for reasons such as its absence on initramfs.  This patch
allows the aesni to function even if the pcbc module is enabled but
not present.

Reported-by: Arkadiusz Miśkiewicz <arekm@maven.pl>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>