iwlegacy: Fix -Wcast-function-type

correct usage prototype of callback in tasklet_init().
Report by https://github.com/KSPP/linux/issues/20

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>