tcp: fix a bitfield constant conversion

../net/ipv4/tcp_timer.c:202:27: warning: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Wsingle-bit-bitfield-constant-conversion]
                icsk->icsk_mtup.enabled = 1;

Change-Id: I7e1b4013ba6a67fad27a611d30f98939ceaa5109