discover: Fix bad check of version string

Clang says this:

discover/device-handler.c:1564:27: warning: size argument in 'strncmp' call is a comparison [-Wmemsize-comparison]
                                        strlen(opt->version) == 0)) {
                                        ~~~~~~~~~~~~~~~~~~~~~^~~~
discover/device-handler.c:1563:5: note: did you mean to compare the result of 'strncmp' instead?
                                strncmp(opt->version, tmp->version,
                                ^

It looks like it's correct. However, we can go one better and drop the
pointless strncmp(foo, bar, strlen(bar)), as this is equivalent to
strcmp(foo, bar).

Signed-off-by: Joel Stanley <joel@jms.id.au>
Reviewed-by: Cyril Bur <cyrilbur@gmail.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>