[PATCH] ISDN: correctly handle isdn_writebuf_stub() errors

isdn_writebuf_stub() forgets to detect memory allocation and uaccess errors.
And when that's fixed, if a error happens the caller will just keep on
looping.

So change the caller to detect the error, and to return it.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Cc: Karsten Keil <kkeil@suse.de>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>