Drop SNDRV_PCM_INFO_DOUBLE

It's a leftover from the rme9652 driver
Only allocate DMA level buffer when compiled with WIP features

We have at least one bug report about a NULL pointer dereference in the
following memset:

   hdspm->level_buffer = snd_sgbuf_get_ptr(&(hdspm->dmaLevelBuffer), 0);
   memset(hdspm->level_buffer, 0, MADIFX_LEVEL_BUFFER_SIZE);

Since level metering is broken anyways, only allocate the DMA level
buffer in the development build.
Provide oldkernel.patch

This patch makes the driver work on ancient kernels, like those included
in Ubuntu 12.04 LTS.
Mention workaround for older kernels