Register interrupt handler when net device is registered. Avoids missing
interrupts if the interrupt mask gets out of sync.

Signed-off-by: Pierre Ossman <redacted>

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The reason this patch is needed for me is that the resume function is
broken. It enables interrupts unconditionally, but the interrupt handler
is only registered when the device is up.

I don't have enough knowledge about the driver to fix the resume
function so this patch will instead make sure that the interrupt handler
is registered at all times (which can be a nice safeguard even when the
resume function gets fixed).