staging: wilc1000: add check for kmalloc allocation failure.

Add a sanity check that wid.val has been allocated, fixes a null
pointer deference on stamac when calling ether_add_copy.

Detected by CoverityScan, CID#1369537 ("Dereference null return value")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>