Don't offset the UAR map twice in mlx5en(4).

The new UAR API already offsets the UAR map pointer the mlx5en(4) is using.
While at it remove some no longer needed variables for keeping track
of the current BF offset.

This fixes a regression issue after the new UAR allocation APIs
were introduced.

MFC after: 1 week
Sponsored by: Mellanox Technologies // NVIDIA Networking