net: delete duplicate dev_set_rx_mode() call

In __dev_open(), it already calls dev_set_rx_mode().
and dev_set_rx_mode() has no effect for a net device which does not have
IFF_UP flag set.

So the call of dev_set_rx_mode() is duplicate in __dev_change_flags().

Signed-off-by: Weiping Pan <panweiping3@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>