While searching for users of spi_async() I found a reference in the ad7877
driver to using it to initiate data transfer from the interrupt handler.
However there is no code for this, instead the interrupt handler is a
threaded handler and uses spi_sync() instead.

Remove the bitrotted comment, though in actual fact the use case mentioned
is a great use for spi_async() since it would cut down on latency handling
the interrupt by saving us a context switch before we start SPI.

This was previously implemented, it was removed in commit b534422b
(Input: ad7877 - switch to using threaded IRQ) for code complexity reasons.
It may be better to revert that commit instead.
Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>