mISDN: positive error return should be negative in mode_hfcmulti()

The error return should be negative.  Its only caller that acts upon its
return, handle_bmsg(), transmits the positive error but can also return
negative errors.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>