staging: wilc1000: restore wilc_spi_dev variable

A recent change to wilc1000 accidentally deleted an important
variable, so we now get a build error when the SPI mode is
selected:

ERROR: "wilc_spi_dev" [drivers/staging/wilc1000/wilc1000.ko] undefined!

This partially reverts the broken commit to put the variable back.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 56293ff2 ("staging: wilc1000: linux_wlan_spi: include header")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/wilc1000/linux_wlan_spi.c

@@ -41,6 +41,7 @@
 
 static u32 SPEED = MIN_SPEED;
 
+struct spi_device *wilc_spi_dev;
 void linux_spi_deinit(void *vp);
 
 static int __init wilc_bus_probe(struct spi_device *spi)