staging: wilc1000: fix a bug when unload driver

kernel crashes when load and unload driver several times. I used git bisect to
track down and found that removing NULL setting caused the panic.
This reverts only related codes of the patch(a4ab1ade).

Fixes: a4ab1ade ("staging: wilc1000: replace drvHandler and hWFIDrv with hif_drv")
Signed-off-by: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>