prism54: isl_ioctl: Remove unused variable 'j'

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

 drivers/net/wireless/intersil/prism54/isl_ioctl.c: In function ‘prism54_update_stats’:
 drivers/net/wireless/intersil/prism54/isl_ioctl.c:166:6: warning: variable ‘j’ set but not used [-Wunused-but-set-variable]

Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Aurelien Alleaume <slts@free.fr>
Cc: Valerio Riedel <hvr@gnu.org>
Cc: "Luis R. Rodriguez" <mcgrof@ruslug.rutgers.edu>
Cc: linux-wireless@vger.kernel.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/20200826093401.1458456-24-lee.jones@linaro.org