fsi/fsi-master-gpio: Implement CRC error recovery

The FSI protocol defines two modes of recovery from CRC errors,
this implements both:

 - If the device returns an ECRC (it detected a CRC error in the
   command), then we simply issue the command again.

 - If the master detects a CRC error in the response, we send
   an E_POLL command which requests a resend of the response
   without actually re-executing the command (which could otherwise
   have unwanted side effects such as dequeuing a FIFO twice).

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Christopher Bostic <cbostic@linux.vnet.ibm.com>
Tested-by: Joel Stanley <joel@jms.id.au>
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Note: This was actually tested by removing some of my fixes, thus
causing us to hit occasional CRC errors during high LPC activity.