[media] media: rc: nuvoton-cir: improve locking in both interrupt handlers

Extend the locking to protect more critical actions like register accesses
in the interrupt handlers.

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