ixgbe: Fix possible null-dereference in error path

In ixgbe_probe, the code at label err_dma can dereference adapter
when it has a NULL value. The check is there to avoid disabling a
disabled device. When adapter is NULL, treat it as if the device
is enabled, because it is enabled in that case.

Signed-off-by: Mark Rustad <mark.d.rustad@intel.com>
Tested-by: Phil Schmitt <phillip.j.schmitt@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>