openssh: if we don't generate a dsa key we shouldn't look for it

openssh: if we don't generate a dsa key we shouldn't look for it

The earlier commit ea119211b21 removed 'dsa' as one of the ssh_host_*_key
types that got generated.  Problem was that it didn't remove that key
as one of the paths that the server looks for by default.  As a
consequence, your log file might fill up with messages like:

    2017-06-01T15:43:07-06:00 openwrt sshd[31929]: error: Could not load host key: /etc/ssh/ssh_host_dsa_key

The patch is forunately trivial.  Don't set the path for the dsa
key file in the server configuration.

Signed-off-by: Philip Prindeville <philipp@redfish-solutions.com>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>