spi: qup: Fix transaction done signaling

Wait to signal done until we get all of the interrupts we are expecting
to get for a transaction.  If we don't wait for the input done flag, we
can be in between transactions when the done flag comes in and this can
mess up the next transaction.

While here cleaning up the code which sets controller->xfer = NULL and
restores it in the ISR. This looks to be some debug code which is not
required.

Signed-off-by: Andy Gross <andy.gross@linaro.org>
Signed-off-by: Varadarajan Narayanan <varada@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>