if_dwc: add detach method

[mhorne]

Summary

It might not be super useful at the moment, but it works.

[manu]

It is useful for testing, thanks ;)

[mhorne]

In D35496#805065, @manu wrote:

It is useful for testing, thanks ;)

Exactly, only I couldn't convince devctl to reattach the device :')

[mmel]

sys/dev/dwc/if_dwc.c

1755

This is slightly racy ->the interrupt should be teardown first, otherwise it may occurs in the middle
of HW deinitialization or ifc destruction.

[mhorne]

Disable and tear down interrupts first in the detach method.

[mhorne]

sys/dev/dwc/if_dwc.c

1755

Thanks. Am I right in thinking that the call to ether_ifdetach() adequately protects us from races with dwc_ioctl()?

Meaning, it should not be possible for dwc_ioctl(SIOCGIFMEDIA) to occur while we are deleting the miibus child.