net/mlx5e: Fix static checker warning of potential pointer math issue

Cited patch have an issue in WARN_ON_ONCE check, with wrong address ranges
are compared. Fix that by changing pointer types from u64* to void*. This
will also make code simpler to read.

In addition mlx5e_hv_vhca_fill_ring_stats can get void pointer, so remove
the unnecessary casting when calling it.

Found by static checker:
drivers/net/ethernet/mellanox/mlx5/core/en/hv_vhca_stats.c:41 mlx5e_hv_vhca_fill_stats()
warn: potential pointer math issue ('buf' is a u64 pointer)

Fixes: cef35af3 ("net/mlx5e: Add mlx5e HV VHCA stats agent")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Eran Ben Elisha <eranbe@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>