ice: tighter control over VSI_DOWN state

The driver had comments to the effect of: This flag should be set before
calling this function. While reviewing code it was found that there were
several violations of this policy, which could introduce hard to find
bugs or races.

Fix the violations of the "VSI DOWN state must be set before calling
ice_down" and make checking the state into code with a WARN_ON.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Gurucharan G <gurucharanx.g@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>