brcm80211: smac: removed MPC related code

The chip init sequence enables MPC (Minimum Power Consumption), but the
driver disables it after that. As there are no interfaces to enable this
mode the related code is unused (member variable wlc->mpc is false).

Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>