be2net: Signal that the device cannot transmit during reconfiguration

While changing the number of interrupt channels, be2net stops adapter
operation (including netif_tx_disable()) but it doesn't signal that it
cannot transmit. This may lead dev_watchdog() to falsely trigger during
that time.

Add the missing call to netif_carrier_off(), following the pattern used in
many other drivers. netif_carrier_on() is already taken care of in
be_open().

Signed-off-by: Benjamin Poirier <bpoirier@suse.com>
Signed-off-by: David S. Miller <davem@davemloft.net>