iwlegacy: debug: Demote seemingly unintentional kerneldoc header

iwlegacy: debug: Demote seemingly unintentional kerneldoc header

This is the only use of kerneldoc in the sourcefile and no
descriptions are provided.

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

 drivers/net/wireless/intel/iwlegacy/debug.c:1373: warning: Function parameter or member 'il' not described in 'il_dbgfs_unregister'

Cc: Stanislaw Gruszka <stf_xl@wp.pl>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Linux Wireless <ilw@linux.intel.com>
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/20200819072402.3085022-10-lee.jones@linaro.org