dmaengine: bcm2835: Allow cyclic transactions without interrupt

dmaengine: bcm2835: Allow cyclic transactions without interrupt

The BCM2835 DMA driver currently requests an interrupt from the
controller regardless whether or not the client has passed in the
DMA_PREP_INTERRUPT flag. This causes unnecessary overhead for cyclic
transactions which do not need an interrupt after each period.

We're about to add such a use case, namely cyclic clearing of the SPI
controller's RX FIFO, so amend the DMA driver to request an interrupt
only if DMA_PREP_INTERRUPT was passed in. Ignore the period_len for
such transactions and set it to the buffer length to make the driver's
calculations work.

Tested-by: Nuno Sá <nuno.sa@analog.com>
Tested-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Lukas Wunner <lukas@wunner.de>
Acked-by: Vinod Koul <vkoul@kernel.org>
Acked-by: Stefan Wahren <wahrenst@gmx.net>
Acked-by: Martin Sperl <kernel@martin.sperl.org>
Cc: Florian Kauer <florian.kauer@koalo.de>
Link: https://lore.kernel.org/r/73cf37be56eb4cbe6f696057c719f3a38cbaf26e.1568187525.git.lukas@wunner.de
Signed-off-by: Mark Brown <broonie@kernel.org>