brcmfmac: fix P2P_DEVICE ethernet address generation

The firmware has a requirement that the P2P_DEVICE address should
be different from the address of the primary interface. When not
specified by user-space, the driver generates the MAC address for
the P2P_DEVICE interface using the MAC address of the primary
interface and setting the locally administered bit. However, the MAC
address of the primary interface may already have that bit set causing
the creation of the P2P_DEVICE interface to fail with -EBUSY. Fix this
by using a random address instead to determine the P2P_DEVICE address.

Cc: stable@vger.kernel.org # 3.10.y
Reported-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Hante Meuleman <hante.meuleman@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieter-paul.giesberts@broadcom.com>
Reviewed-by: Franky Lin <franky.lin@broadcom.com>
Signed-off-by: Arend van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>