crypto: sha1-mb - Remove pointless cast

Since kzalloc() returns a void pointer, we don't need to cast the
return value in arch/x86/crypto/sha-mb/sha1_mb.c::sha1_mb_mod_init().

Signed-off-by: Firo Yang <firogm@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>