brcmfmac: assure P2P discovery is disabled when setting P2P_DEVICE mac address

In order to provision the P2P_DEVICE mac address using p2p_da_override iovar
the discovery interface must be disabled. On some targets setting the mac
address failed so disable the discovery interface to be certain.

Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Reviewed-by: Franky (Zhenhui) Lin <frankyl@broadcom.com>
Reviewed-by: Daniel (Deognyoun) Kim <dekim@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>