staging: gasket: gasket_mmap return error instead of valid BAR index

When offset to be mapped matches both a BAR region and a coherent mapped
region return an error as intended, not the BAR index.
Signed-off-by: Simon Que <sque@chromium.org>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/gasket/gasket_core.c

@@ -1627,7 +1627,7 @@ static int gasket_mmap(struct file *filp, struct vm_area_struct *vma)
 			"0x%lx",
 			raw_offset);
 		trace_gasket_mmap_exit(bar_index);
-		return bar_index;
+		return -EINVAL;
 	}
 
 	vma->vm_private_data = gasket_dev;