selftests/net: timestamping: Fix bind_phc check

timestamping checks socket options during initialisation. For the field
bind_phc of the socket option SO_TIMESTAMPING it expects the value -1 if
PHC is not bound. Actually the value of bind_phc is 0 if PHC is not
bound. This results in the following output:

SIOCSHWTSTAMP: tx_type 0 requested, got 0; rx_filter 0 requested, got 0
SO_TIMESTAMP 0
SO_TIMESTAMPNS 0
SO_TIMESTAMPING flags 0, bind phc 0
   not expected, flags 0, bind phc -1

This is fixed by setting default value and expected value of bind_phc to
0.

Fixes: 2214d703 ("selftests/net: timestamping: support binding PHC")
Signed-off-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>