ice: write register with correct offset

The VF_MBX_ARQLEN register array is per-PF, not global, so we should not
use the absolute VF ID as an index. Instead, use the per-PF VF ID.

This fixes an issue with VFs on PFs other than 0 not seeing reset.

Signed-off-by: Mitch Williams <mitch.a.williams@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>