brcmfmac: Don't control mpc setting during scan operation

Instead of controlling mpc setting during scan operation, initialize
mpc setting and then let firmware take care of it.

Reviewed-by: Pieter-Paul Giesberts <pieterpg@broadcom.com>
Reviewed-by: Hante Meuleman <meuleman@broadcom.com>
Signed-off-by: Daniel Kim <dekim@broadcom.com>
[arend@broadcom.com: keep mpc setting for bcm4329]
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>