brcm80211: remove sparse warning in fullmac debug function

The debug function did a write operation which required a different
pointer type resulting in a sparse warning.

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