media: rtl28xxu: fix idle handling

Currently the IR receiver is put into idle mode after each interrupt.
However longer IR like NEC can be distributed over multiple interrupts,
so putting the IR into idle in the middle of such a message will break
decoding.

Every IR message has a trailing space of 0xbf (one less than
IR_IDE_LEN{0,1} which is programmed to 0xc0). So, set the timeout to that
value and rc-core will put the IR receiver into idle mode automatically.

Thanks to Jan Pieter van Woerkom <jp@jpvw.nl> for providing the hardware.

Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>