V4L/DVB (13097): cx23885: Complete CX23888 IR subdev implementation for Rx & almost for Tx

This change completes the v4l2_subdev implementation for IR receive for the
IR controller built into the CX23888.

This changes almost completes the IR transmit side also, but doesn't.  Instead
notes in the comments describe what needs to be done for IR Tx to work in the
subdevice implementation.

The current Tx behavior is skeletal and benign.  If left alone, it does nothing.
It will only ever generate a Tx interrupt on Tx init by a caller or when the
tx_write() method is called.  The ISR, when called, will then disable the Tx
FIFO service interrupt.

Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>