staging: wilc1000: fix rmmod failure

This patch fixes rmmod failure. wilc->firmware needs to be set to NULL because
it is used again to check firmware is released when module exit.

Fixes: 8b8ad7bc ("staging: wilc1000: rename wilc_firmware in the struct wilc")
Signed-off-by: NGlen Lee <glen.lee@atmel.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wilc1000/linux_wlan.c

@@ -506,6 +506,7 @@ static int wilc1000_firmware_download(struct net_device *dev)
 	PRINT_D(INIT_DBG, "Freeing FW buffer ...\n");
 	PRINT_D(INIT_DBG, "Releasing firmware\n");
 	release_firmware(wilc->firmware);
+	wilc->firmware = NULL;
 
 	PRINT_D(INIT_DBG, "Download Succeeded\n");