net: dsa: return success if there was nothing to do

Clang static analysis reports this representative issue
dsa.c:486:2: warning: Undefined or garbage value
  returned to caller
  return err;
  ^~~~~~~~~~

err is only set in the loop.  If the loop is empty,
garbage will be returned.  So initialize err to 0
to handle this noop case.

Signed-off-by: Tom Rix <trix@redhat.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>