mlx4_core: Fix one more static exported function

Commit 22c8bff6 ("mlx4_core: Exported functions can't be static")
fixed most of this up, but forgot about mlx4_is_slave_active().  Fix
this one too.

Cc: <stable@vger.kernel.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>