dev: Fixing EtherDevice stats initialization order

dev: Fixing EtherDevice stats initialization order

Previously, the stat `totalBandwidth` is initialized before
`txBandwidth` and `rxBandwidth`. However, `totalBandwith` is of
 type Stats::Formula and `totalBandwidth = txBandwidth + rxBandwidth`.
Therefore, `totalBandwidth` should be initialized after the other two.

This change fixes the variable and stats initialization order accordingly.

The bug was reported here:  https://github.com/gem5/gem5/commit/3db48cbbc6e475592e6608b52a870d92ac2214aa#commitcomment-46094633.

Jira: https://gem5.atlassian.net/browse/GEM5-894

Change-Id: I2c7cc4120df672edf15b9a3ab6becc0bbebb778b
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39395
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>