spi: dw: Discard IRQ threshold macro

The macro has been unused since a half of FIFO length was defined to be a
marker of the IRQ. Let's remove it definition.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20200920112914.26501-2-Sergey.Semin@baikalelectronics.ru


Signed-off-by: Mark Brown <broonie@kernel.org>