ice: remove redundant max_vsi_num variable

It is a leftover from previous implementation. Accidentally it wasn't
removed. Do it now.

Commit that has removed it:
commit c1e5da5d

 ("ice: improve switchdev's slow-path")

Reviewed-by: Wojciech Drewek <wojciech.drewek@intel.com>
Reviewed-by: Piotr Raczynski <piotr.raczynski@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>