brcmfmac: allow scanning to be suppressed in the driver

During the DHCP protocol exchange it is benificial to suppress
scan requests which may decrease time to complete DHCP protocol.

Reviewed-by: Hante Meuleman <meuleman@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>