mISDN: Add different different timer settings for hfc-pci

     - Poll-timer can now be set from 8 to 256 samples, depending on your kernel.
     - If default or 128 is used, the normal controller's clock is used as before.
       Usage: modprobe hfcpci poll=XXX
     - Added some debug code for dsp buffer size. (CMX_DELAY_DEBUG)

    Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
    Signed-off-by: Karsten Keil <kkeil@suse.de>