staging: gdm72xx: fix reference counting in gdm_wimax_event_init

This patch fixes the commit "staging/gdm72xx: cleanup little at
gdm_wimax_event_rcv" (8df858ea),
which mishandles the reference counting of wm_event.

Signed-off-by: Ben Chan <benchan@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>