dma: edma: Find missed events and issue them

In an effort to move to using Scatter gather lists of any size with
EDMA as discussed at [1] instead of placing limitations on the driver,
we work through the limitations of the EDMAC hardware to find missed
events and issue them.

The sequence of events that require this are:

For the scenario where MAX slots for an EDMA channel is 3:

SG1 -> SG2 -> SG3 -> SG4 -> SG5 -> SG6 -> Null

The above SG list will have to be DMA'd in 2 sets:

(1) SG1 -> SG2 -> SG3 -> Null
(2) SG4 -> SG5 -> SG6 -> Null

After (1) is succesfully transferred, the events from the MMC controller
donot stop coming and are missed by the time we have setup the transfer
for (2). So here, we catch the events missed as an error condition and
issue them manually.

In the second part of the patch, we make handle the NULL slot cases:
For crypto IP, we continue to receive events even continuously in
NULL slot, the setup of the next set of SG elements happens after
the error handler executes. This is results in some recursion problems.
Due to this, we continously receive error interrupts when we manually
trigger an event from the error handler.

We fix this, by first detecting if the Channel is currently transferring
from a NULL slot or not, that's where the edma_read_slot in the error
callback from interrupt handler comes in. With this we can determine if
the set up of the next SG list has completed, and we manually trigger
only in this case. If the setup has _not_ completed, we are still in NULL
so we just set a missed flag and allow the manual triggerring to happen
in edma_execute which will be eventually called. This fixes the above
mentioned race conditions seen with the crypto drivers.

[1] http://marc.info/?l=linux-omap&m=137416733628831&w=2

Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>

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