spi: tegra20-slink: Put device into suspend on driver removal

pm_runtime_disable() cancels all pending power requests, while they
should be completed for the Tegra SPI driver. Otherwise SPI clock won't
be disabled ever again because clk refcount will become unbalanced.
Enforce runtime PM suspension to put device into expected state before
driver is unbound and device's RPM state is reset by driver's core.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20211023225951.14253-1-digetx@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>