logging: adsp hda backend refinements and additional test

Additional testing showed that when using printk the logger would start
sticking and spitting out nulls which is wrong. This made it appear as if
the firmware had locked up. The issue seems to have been caused by the
initial ipc message to read all the dma buffers on the host.

Removing that, the issue seems to have been solved.

This also improves the test case to ensure printk with LOG_PRINTK=y
works as expected. It also adds a last log message between some
timeouts of the flush timer length to ensure the padding and timer
flush are working properly.

Signed-off-by: Tom Burdick <thomas.burdick@intel.com>