dmaengine: Remove dma_request_slave_channel_compat() wrapper

At its original introduction, dma_request_slave_channel_compat() used a
wrapper, to accommodate filter functions that modify the mask passed.
Filter functions can no longer modify masks, and the mask parameter was
made const in commit a53e28da ("dma: Make the 'mask' parameter
of __dma_request_channel const") consecutively.

Hence remove the wrapper, and rename __dma_request_slave_channel_compat()
to dma_request_slave_channel_compat(), to get rid of one more function
name starting with a double underscore.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200121093311.28639-3-geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>