tests: net: ethernet_mgmt: Don't recalculate deltaBW with no link
tests: net: ethernet_mgmt: Don't recalculate deltaBW with no link

Fixes #9462

Coverity ID: 187670

Attempts to recalculate deltaBandwidth with no link lead to division by
zero.

Signed-off-by: Tomasz Gorochowik <265c7c25eca9aa9127910457b0aea7e793d329f1@antmicro.com>

GiulianoFranchetto/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,nashif/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,nashif/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,punitvara/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,punitvara/zephyr