bluetooth: make bnep_sock_cleanup() return void

bnep_sock_cleanup() always returns 0 and its return value isn't used
anywhere in the code.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>