net: sock: Use USEC_PER_SEC macro instead of literal 1000000

The USEC_PER_SEC is used once in sock_set_timeout as the max value of
tv_usec. But there are other similar codes which use the literal
1000000 in this file.
It is minor cleanup to keep consitent.

Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>