iavf: Fix missing check for running netdev

The driver was queueing reset_task regardless of the netdev
state.

Do not queue the reset task in iavf_change_mtu if netdev
is not running.

Fixes: fdd4044f

 ("iavf: Remove timer for work triggering, use delaying work instead")
Signed-off-by: Slawomir Laba <slawomirx.laba@intel.com>
Signed-off-by: Phani Burra <phani.r.burra@intel.com>
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Mateusz Palczewski <mateusz.palczewski@intel.com>
Tested-by: Konrad Jankowski <konrad0.jankowski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>