Rearrange interrupt handler a bit and remove forever loop.

Previously dc(4) always checked whether there is pending interrupts
and this consumed a lot of CPU cycles in interrupt handler. Limit
the number of processing for TX/RX frames to 16. Also allow sending
frames in the loop not to starve TX under high RX load.
Reading DC_ISR register should be protected with driver lock,
otherwise interrupt handler could be run(e.g. link state change)
before the completion of dc_init_locked().

While I'm here remove unneeded code.

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