net/mlx4_en: Fix uninitialized use of 'port_up' in mlx4_en_set_channels()

Function mlx4_en_set_channels() stops running ports before performing the
requested action. In that case local variable 'port_up' is set so that the
port is restarted at the end of the function, however, in case the port was
not stopped, variable 'port_up' is left uninitialized and the behaviour is
undetermined. Detected by Coverity - CID 751497.

Signed-off-by: Christian Engelmayer <cengelma@gmx.at>
Acked-By: Amir Vadai <amirv@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>