net: icmp: Fix return values of ICMP error send function

The return code from net_icmpv{4|6}_send_error() was not correct
if the error message could be sent. Now 0 is returned if sending
succeed, and <0 otherwise.

Change-Id: Iff67f097a9d9519c9f11d4cbc9cf428a7c74ec1b
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>