net: statistics: Expose relevant information through net mgmt API

User application can request the information it wants via the generic
net_mgmt() call, following the NET_REQUEST_STATS_* codes.

Change-Id: Ia9e7d318cf11b7bf8bfaf1ad63c8c985be846cc1
Signed-off-by: Tomasz Bursztyka <tomasz.bursztyka@linux.intel.com>