b43legacy: Fix -Wcast-function-type

[ Upstream commit 475eec112e4267232d10f4afe2f939a241692b6c ]

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



Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>