net/mlx4_core: Do not BUG_ON during reset when PCI is offline

The PCI channel could go offline during reset due to EEH.  Don't bug on in
this case, the error is recoverable.

Fixes: f6bc11e4 ('net/mlx4_core: Enhance the catas flow to support device reset')
Signed-off-by: Daniel Jurgens <danielj@mellanox.com>
Reviewed-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>