mISDN: Use kernel_{send,recv}msg instead of open coding

Reducing the number of direct users of sock_{recv,send}msg.
Modified version to match the latest context.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: David S. Miller <davem@davemloft.net>