kernel/wr_nic/device.c: parent of netdev is pdev (fixes oops)

This fixes an oops when programs access ethtool methods, especially
wrn_get_drvinfo. It can be reproduced by a simple run of "irqbalance",
which I never did in my development machine.

Reported-by: Grzegorz Daniluk <grzegorz.daniluk@gmail.com>
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>

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