Don't call destroy_dev it a channel has children. vchan creation doesn't lead to /dev entry creation if the new vchan is the first child of a channel, This fix a panic that happens when loading a sound driver module, creating vchans and unloading the driver.
Don't call destroy_dev it a channel has children.
vchan creation doesn't lead to /dev entry creation if the new vchan is the
first child of a channel,
This fix a panic that happens when loading a sound driver module, creating
vchans and unloading the driver.

Approved by:	cg
MFC after:	3 days

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