staging: most: hdm-dim2: monitor atx DBR space

This patch implements a workaround for a DIM2 issue where the device
macro, in case the asynchronous channel sends data (to MOST), sporadically
duplicates the synchronous blocks with a size of half the synchronous DBR
buffer.

The patch monitors the size of the unused asynchronous Tx DBR memory
(that includes the ping and pong sizes) to prevent the potential DBR
overflow for the asynchronous Tx DBR buffer.

The patched DIM2 HDM expects that the platform driver delivers the 2nd
platform irq (index 1) as the mlb_int of the DIM2 macro.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>