usertools: fix device binding with kernel tools

usertools: fix device binding with kernel tools

The following sequence of operation gives error in binding devices
1) Bind a device using dpdk-devbind.py
2) Unbind the device using kernel tools(/sys/bus/pci/device/driver/unbind)
3) Bind the device using kernel tools(/sys/bus/pci/driver/new_id and
/sys/bus/pci/driver/bind)

The bind failure was due to cached driver name in 'driver_override'.
Fix it by writing 'null' to driver_override just after binding a
device so that any method of binding/unbinding can be used.

Fixes: 2fc350293570 ("usertools: use optimized driver override scheme to bind")

Reported-by: Lijuan A Tu <lijuanx.a.tu@intel.com>
Signed-off-by: Guduri Prathyusha <gprathyusha@caviumnetworks.com>