[media] media: rc: nuvoton: ignore spurious interrupt when logical device is being disabled

When removing module nuvoton-cir I get a fifo overrun warning.
It turned out to be caused by a spurious interrupt when the logical CIR
device is being disabled (although no interrupt source bit being set).
Reading the interrupt status register returns 0xff, therefore the fifo
overrun bit is mistakenly interpreted as being set.

Fix this by ignoring interrupts when interrupt source and status register
reads return 0xff.

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