net: eth-uclass: Fix for DM USB ethernet support

When a USB ethernet device is halted, the device driver is removed. When
this happens the uclass private memory is freed and uclass_priv is set to
NULL. This causes a data abort when uclass_priv->state is then set to
ETH_STATE_PASSIVE.

Fix it by checking if uclass_priv is NULL before setting uclass_priv->state

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>