net: mctp: Remove unnecessary cast in mctp_cb

The void * cast in mctp_cb is unnecessary as it's already been done
at the start of the function.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Jeremy Kerr <jk@codeconstruct.com.au>

Link: https://patch.msgid.link/Z9PwOQeBSYlgZlHq@gondor.apana.org.au
Signed-off-by: Paolo Abeni <pabeni@redhat.com>