brcmfmac: Rework p2p attach, use single method for p2p dev creation.

When module param p2pon is used a p2p device is created at init.
This patch reworks how this is done by using the same method as
for a dynamically (by user space) created p2p device.

Reviewed-by: Arend Van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Signed-off-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>