Instead of one request line member let's use both source and destination ones.
Usually we have no such hardware except Atmel MMC controller found on AVR32
platform (see arch/avr32/mach-at32ap/at32ap700x.c and
drivers/mmc/host/atmel-mci.c).

This patch removes slave_id usage since it'll be removed from the generic
structure in later. This breaks the non-ACPI / non-DT cases for the users of
the driver, i.e. SPI and HSUART. However, these cases mean only PCI enumerated
devices for now, which is anyway broken (considering more than one DMA
controller in the system) and this patch series is intended to fix that
eventually.

The ACPI and DT cases shall be aware of the channel direction when setting
request lines, but this is a minor problem that would be addressed in future.
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>