show: pass NULL parameter to config_key_has_cap() instead of uninitialized cg.

It doesn't make sense to pass a cg to this config_key_has_cap() call
as it is trying to retrieve the device's global SR_CONF_LIMIT_SAMPLES.

This fixes the following warning:

show.c: In function ‘show_dev_detail’:
show.c:392:8: warning: ‘cg’ may be used uninitialized in this function [-Wmaybe-uninitialized]
     && config_key_has_cap(driver, sdi, cg, key, SR_CONF_LIST)) {
        ^