ucc_geth: NAPI-related bug fixes

Based partly on the gianfar driver, this patch fixes several
bugs which were causing NAPI to be completely unusable.
* An IRQ is still needed in NAPI, to kick off NAPI task,
  and for Tx processing.  Request the IRQ.
* If rx_work_limit = 0 we are not complete.
* While running Rx NAPI processing we must mask Rx events,
  including Rx busy.
* ucc_geth_rx function does not need a lock.
  Could lead to deadlock in NAPI case.
* There's no need to loop reading ucce multiple times in the ISR,
  so while adding the call to schedule NAPI which was not there,
  simplify the event processing into if-else format.
* Rx Busy now kicks off NAPI processing, while still
  being counted as an error.

Signed-off-by: Michael Reiss <michael.f.reiss@freescale.com>
Signed-off-by: Michael Barkowski <michael.barkowski@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>