[PLIP] Check cmd in plip_ioctl

Hi Jeff:

I received a bug report that a PLIP interface was incorrectly identified
as wireless because plip_ioctl did not check what the value of cmd is
before processing the request.

This patch fixes exactly that.

Cheers,
-- 
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Email:  Herbert Xu ~{PmV>HI~} [off-list ref]
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