ice: refactor ice_vsi_is_vlan_pruning_ena

As this method became static, and is already called
with check for vsi being non-null, an unnecessary check along
with superfluous parentheses is removed.

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>