brcm80211: use container_of to resolve brcms_phy from brcms_phy_pub

Use container_of instead of casting first structure member.

Compiled but untested.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>