net/mlx5: Fix function calculation for page trees

The function calculation always results in a value of 0. This works
generally, but when the release all pages feature is enabled it will
result in crashes.

Fixes: 0aa12847 ("net/mlx5: Maintain separate page trees for ECPF and PF functions")
Signed-off-by: Daniel Jurgens <danielj@nvidia.com>
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>