iwlwifi: remove a parameter (dest) from *_fill_beacon_frame() methods.

This patch removes a parameter (dest) from iwl_fill_beacon_frame() (iwl-agn.c)
 and from iwl3945_fill_beacon_frame(). (iwl-3945.c,iwl-3945.h)

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>