MFC r330649:
Add support for per priority flow control, PFC, to mlx5en(4).

Add support for PFC and implement reading the per priority statistics
using the sysctl(8) interface. PFC is used together with VLAN priority
and can be enabled and disabled on a per priority basis.

Global pause frames and PFC are incompatible features and surrounding
logic has been added to warn the user about misconfiguration.

Update relevant mlx5core APIs for PFC configuration.

Sponsored by: Mellanox Technologies