[media] media: rc: nuvoton: remove usage of b_idx in nvt_get_rx_ir_data

The call to nvt_get_rx_ir_data and nvt_process_rx_ir_data from
the ISR is protected with spinlock nvt->lock. Therefore it's
guaranteed that nvt->pkts is 0 when entering nvt_get_rx_ir_data
(as nvt->pkts is set to 0 at the end of nvt_process_rx_ir_data).
Having said that we can remove b_idx.

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