We add an IRQ handler that implements a state-machine for PIO-mode and data
structures for walking the scatter-gather list. The IRQ handler is called in
succession both when data is available to read or next data can be sent for
processing. This process continues till the entire in/out SG lists have been
walked. Once the SG-list has been completely walked, the IRQ handler schedules
the done_task tasklet.

Also add a useful macro that is used through out the IRQ code for a common
pattern of calculating how much an SG list has been walked.  This improves code
readability and avoids checkpatch errors.
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>