About IFNET_PCP_NONE

Hi Konstantin,


I was just about going to open a PR for https://reviews.freebsd.org/D39536 and
realized I might made wrong assumption.

I thought IFNET_PCP_NONE is something like IEEE8021Q_PCP_BE but I second why not
use IEEE8021Q_PCP_BE but a new const IFNET_PCP_NONE.

So despite its naming IFNET_PCP_NONE, is it actually a flag to let specific interface
completely bypass (disable) PCP processing?

The const IFNET_PCP_NONE is defined in sys/net/if.h with 
```
#define IFNET_PCP_NONE 0xff   /* PCP disabled */
```


Best regards,
Zhenlei