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: Glen Lee <glen.lee@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>