examples/l3fwd-power: fix metrics divisions

examples/l3fwd-power: fix metrics divisions

6 issues caught by Coverity 343465
* Possible divide by zero on 3 lines
* Convert to float then back to int, losing precision on 3 lines

This patch modifies the code so that it only assigns calculated
values if the divisor is > 0, otherwise sets metrics to zero.
Also removes the un-needed round() function.

Coverity issue: 343465
Fixes: 609e79841fcf ("examples/l3fwd-power: add telemetry mode")
Signed-off-by: David Hunt <david.hunt@intel.com>