misc: mic: scif: fix potential double free of scif_dev

_scif_init() free scif_dev in the free_sdev erro path,
but _scif_exit will free it again when module exit, it
cause BUG_ON issue,

kernel BUG at mm/slub.c:3944!
invalid opcode: 0000 [#1] SMP KASAN PTI

Set scif_dev to NULL in scif_destroy_scifdev() to fix it.

Cc: Sudeep Dutt <[e-mail]>
Cc: Ashutosh Dixit <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Greg Kroah-Hartman <[e-mail]>
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Kefeng Wang <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/misc/mic/scif/scif_main.c

···
 static void scif_destroy_scifdev(void)
 {
 	kfree(scif_dev);
+	scif_dev = NULL;
 }
 
 static int scif_probe(struct scif_hw_dev *sdev)