net: b44: set pause params only when interface is up

b44_free_rings() accesses b44::rx_buffers (and ::tx_buffers)
unconditionally, but b44::rx_buffers is only valid when the
device is up (they get allocated in b44_open(), and deallocated
again in b44_close()), any other time these are just a NULL pointers.

So if you try to change the pause params while the network interface
is disabled/administratively down, everything explodes (which likely
netifd tries to do).

Link: https://github.com/openwrt/openwrt/issues/13789


Fixes: 1da177e4 (Linux-2.6.12-rc2)
Cc: stable@vger.kernel.org
Reported-by: Peter Münster <pm@a16n.net>
Suggested-by: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Vaclav Svoboda <svoboda@neng.cz>
Tested-by: Peter Münster <pm@a16n.net>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Peter Münster <pm@a16n.net>
Reviewed-by: Michael Chan <michael.chan@broadcom.com>
Link: https://lore.kernel.org/r/87y192oolj.fsf@a16n.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>