platform/x86: apple-gmux: return -EFAULT if copy fails

The copy_to/from_user() functions return the number of bytes remaining
to be copied, but we want to return -EFAULT to the user.

Fixes: ce3fef2eb235 ("platform/x86: apple-gmux: add debugfs interface")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Reviewed-by: Orlando Chamberlain <orlandlch.dev@gmail.com>
Link: https://lore.kernel.org/r/0bdfa8c2-cb22-4bec-8773-584060613043@kili.mountain


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>