[bwn] Enable PHY-LP 5GHz support for the one NIC I have tested it on so far.

After perusing the PHY-LP code (don't ask why; honest) I discovered that
it /has/ 5GHz support - but it's not ever used. I found one NIC - a
BCM4312 w/ pci id 0x4315 - which advertised dual-band PHY-LP support.

Turns out it works.

Whilst here, move up the support bit logging code so I can use it
to debug this.

Tested:

• BCM4312 (pci id 0x4315); 5GHz STA operation