dsa: Remove unnecessary exports

I mistakenly exported functions from slave.c that are only called from
dsa.c, part of the same module.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Acked-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: David S. Miller <davem@davemloft.net>