staging: brcm80211: add braces to SI_INFO macro definition

The additional braces allow the casted parameter to be indirected
immediately. Here is an example to clarify:
	x = SI_INFO(y);    => z = SI_INFO(y)->field_a;
	z = x->field_a;

Signed-off-by: Arend van Spriel <arend@broadcom.com>
Reviewed-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>