staging: brcm80211: remove unnecessary cast in wlc_d11hdrs_mac80211

memset prototype specifies a void pointer as buffer. Conversion from
any pointer type to void pointer does not require an explicit cast.

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