staging: atomisp: potential underflow in atomisp_get_metadata_by_type()

md_type is an enum.  On my tests, GCC treats it as unsigned but
according to the C standard it's an implementation dependant thing so we
should check for negatives.

Fixes: a49d2536 ("staging/atomisp: Add support for the Intel IPU v2")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/media/atomisp/pci/atomisp2/atomisp_cmd.c

@@ -3004,7 +3004,7 @@ int atomisp_get_metadata_by_type(struct atomisp_sub_device *asd, int flag,
 		return 0;
 
 	md_type = md->type;
-	if (md_type >= ATOMISP_METADATA_TYPE_NUM)
+	if (md_type < 0 || md_type >= ATOMISP_METADATA_TYPE_NUM)
 		return -EINVAL;
 
 	/* This is done in the atomisp_buf_done() */