Spinlock nvt_lock is a member of struct nvt_dev and there's no need
to prefix it with nvt_. So remove this prefix.

[mchehab@s-opensource.org: change the prefix also at the open function,
 as the patch removing it were not applied (yet?)]
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>