Change the driver stats to what they really are: unsigned values.

When pmcstat exits after some samples were dropped, give the user an
idea of how many were lost. (Granted, these are global numbers, but
they may still help quantify the scope of the loss.)

Differential Revision: https://reviews.freebsd.org/D4123
Approved by: gnn (mentor)
MFC after: 1 month
Sponsored by: Juniper Networks