event/sw: fix get of single xstat value

event/sw: fix get of single xstat value

This commit allows the xstats_get() API to return just a single
value based on its ID. Previously, the "ret_n_lt_stats" value
set to 1 ensured a check to take place that the array was larger
than the available xstats.

The xstats_get() API allows retriving of individual stats - hence
this check should be removed - by setting "ret_n_lt_stats" to zero.

Fixes: c1ad03df7ad5 ("event/sw: support xstats")

Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>