mISDN: Add allocation of recvbuf[1500] at run time to reduce stack size

Before: 1656 bytes on i386, now 164.
Modified by Andreas Eversberg <andreas@eversberg.eu>

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>