p54: memset(0) whole array

gcc 7 complains:
drivers/net/wireless/intersil/p54/fwio.c: In function 'p54_scan':
drivers/net/wireless/intersil/p54/fwio.c:491:4: warning: 'memset' used with length equal to number of elements without multiplication by element size [-Wmemset-elt-size]

Fix that by passing the correct size to memset.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Christian Lamparter <chunkeey@googlemail.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Acked-by: Christian Lamparter <chunkeey@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>