test/bonding: fix after hiding ethdev internal structures

test/bonding: fix after hiding ethdev internal structures

link bounding auto-test internally creates emulated ethdev.
Some tests change Rx/Tx functions of this emulated device on the fly:
by directly modifying rte_eth_dev fields and without doing stop/start
for these devices.
As now ethdev uses rte_eth_fp_ops[] for fast-path functions, these
direct changes doesn't make expected effect.
Fix the problem by guarding fast-path functions changes with
rte_eth_dev_stop()/rte_eth_dev_start().

Fixes: 7a0935239b9e ("ethdev: make fast-path functions to use new flat array")
Reported-by: Lewei Yang <leweix.yang@intel.com>
Signed-off-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
Tested-by: Ferruh Yigit <ferruh.yigit@intel.com>