Add support for buffer parameter manipulations in mlx5en(4).

The following sysctls are added:
dev.mce.N.conf.qos.cable_length
dev.mce.N.conf.qos.buffers_size
dev.mce.N.conf.qos.buffers_prio

Submitted by: kib@
MFC after: 3 days
Sponsored by: Mellanox Technologies