mISDN: Fix handling of receive buffer size in L1oIP

The size of receive buffer pointer was used to get size of
receive buffer instead of recvbuf_size itself, so only 4/8
bytes could be transfered.

This is a regression to 2.6.30 introduced by commit
8c90e11e3543d7de612194a042a148caeaab5f1d ("mISDN: Use
kernel_{send,recv}msg instead of open coding")

Signed-off-by: Andreas Eversberg <andreas@eversberg.eu>
Signed-off-by: Karsten Keil <keil@b1-systems.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>