P2P: Derive group interface name bit more sensibly

P2P: Derive group interface name bit more sensibly

The interface name for the P2P group interface is derived from the
P2P management interface. When the P2P management interface is a
P2P Device interface, i.e., p2p-dev-wlanX, the name for the group
interface is abbreviated to p2p-X (X being group index). When the
P2P management interface starts with p2p-dev- use its postfix
instead. So P2P management interface p2p-dev-wlan3 results in group
interface name p2p-wlan3-0.

Signed-hostap: Arend van Spriel <arend@broadcom.com>