tcp: Remove runtime check that can never be true.

GCC even warns about it, as reported by Andrew Morton:

net/ipv4/tcp.c: In function 'do_tcp_getsockopt':
net/ipv4/tcp.c:2544: warning: comparison is always false due to limited range of data type

Signed-off-by: David S. Miller <davem@davemloft.net>