brcmfmac: move attach and detach functions in wl_cfg80211.c

Preparing for another patch move the functions in separate commit.

Reviewed-by: Hante Meuleman <meuleman@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>