iwlegacy: common: Remove set but not used variable 'len'

iwlegacy: common: Remove set but not used variable 'len'

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

 drivers/net/wireless/intel/iwlegacy/common.c: In function ‘il_enqueue_hcmd’:
 drivers/net/wireless/intel/iwlegacy/common.c:3126:6: warning: variable ‘len’ set but not used [-Wunused-but-set-variable]

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/20200814113933.1903438-15-lee.jones@linaro.org