spi: dw: Fix spi registration for controllers overriding CS

spi: dw: Fix spi registration for controllers overriding CS

When SPI DW memory ops support was introduced, there was a check for
excluding controllers which supplied their own CS function. Even so,
the mem_ops pointer is *always* presented to the SPI core.

This causes the SPI core sanity check in spi_controller_check_ops() to
refuse registration, since a mem_ops pointer is being supplied without
an exec_op member function.

The end result is failure of the SPI DW driver on sparx5 and similar
platforms.

The fix in the core SPI DW driver is to avoid presenting the mem_ops
pointer if the exec_op function is not set.

Fixes: 7d4368680c5a (spi: dw: Add memory operations support)
Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Acked-by: Serge Semin <fancer.lancer@gmail.com>
Link: https://lore.kernel.org/r/20201120213414.339701-1-lars.povlsen@microchip.com
Signed-off-by: Mark Brown <broonie@kernel.org>