spi: altera: Switch to SPI core transfer queue management

The Altera SPI driver currently uses the spi-bitbang infrastructure for
transfer queue management, but non of the bitbang functionality itself.
This is because when the driver was written this was the only way to not
have to do queue management in the driver itself.

Nowadays transfer queue management is available from the SPI driver core
itself and using the bitbang infrastructure just adds an additional level
of indirection.

Switch the driver over to using the core queue management directly.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>