ethtool-util: fix setting advertising link modes

ethtool-util: fix setting advertising link modes

Fixes a regression caused by d307410327d14398cb60b72db9d0034b12950a72.

The link_mode_masks flex array in struct ethtool_link_settings contains
three packed arrays, and the length of each array is given by
link_mode_masks_nwords field:
```
        __u32   link_mode_masks[];
        /* layout of link_mode_masks fields:
         * __u32 map_supported[link_mode_masks_nwords];
         * __u32 map_advertising[link_mode_masks_nwords];
         * __u32 map_lp_advertising[link_mode_masks_nwords];
         */
```
Hence, we cannot use the received data as is through the union, but need
to shift the array to make each map accessible through the union.