spi: dw: Move num-of retries parameter to the header file

The parameter will be needed for another wait-done method being added in
the framework of the SPI memory operation modification in a further
commit.

Signed-off-by: Serge Semin <Sergey.Semin@baikalelectronics.ru>
Link: https://lore.kernel.org/r/20201007235511.4935-16-Sergey.Semin@baikalelectronics.ru
Signed-off-by: Mark Brown <broonie@kernel.org>