staging: rtl8188eu: use netdev routines for private data

This driver implements its own routines to allocate, access and free the
private data of its net_device. Use the functionality from the networking
core instead.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210321162009.15447-1-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>