[media] media: rc: nuvoton: eliminate nvt->tx.lock

Using a separate spinlock to protect access to substruct tx of struct
nvt_dev doesn't provide any actual benefit. We can use spinlock
nvt_lock to protect all access to struct nvt_dev and get rid of
nvt->tx.lock.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>