cifs: use echo_interval even when connection not ready.

When the tcp connection is not ready to send requests,
we keep retrying echo with an interval of zero.

This seems unnecessary, and this fix changes the interval
between echoes to what is specified as echo_interval.

Signed-off-by: Shyam Prasad N <sprasad@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>