Fixes the following W=1 kernel build warning(s):

 drivers/net/wireless/broadcom/b43legacy/main.c: In function ‘b43legacy_interrupt_tasklet’:
 drivers/net/wireless/broadcom/b43legacy/main.c:1344:3: warning: suggest braces around empty body in an ‘if’ statement [-Wempty-body]

Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Martin Langer <martin-langer@gmx.de>
Cc: Stefano Brivio <stefano.brivio@polimi.it>
Cc: Michael Buesch <m@bues.ch>
Cc: van Dyk <kugelfang@gentoo.org>
Cc: Andreas Jaggi <andreas.jaggi@waterwave.ch>
Cc: linux-wireless@vger.kernel.org
Cc: b43-dev@lists.infradead.org
Cc: netdev@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200814113933.1903438-18-lee.jones@linaro.org