mISDN: Reduce stack size in dsp_cmx_send()

Reduce stack size memory footprint of mISDN_dsp.
(From 1468 bytes for dsp_cmx_send on i386 down to 44).

Signed-off-by: Frank Seidel <frank@f-seidel.de>
Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>