spi: tegra20-slink: Improve runtime PM usage

spi: tegra20-slink: Improve runtime PM usage

The Tegra SPI driver supports runtime PM, which controls the clock
enable state, but the clk is also enabled separately from the RPM
at the driver probe time, and thus, stays always on. Fix it.

Runtime PM now is always available on Tegra, hence there is no need to
check the RPM presence in the driver anymore. Remove these checks.

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Link: https://lore.kernel.org/r/20210731192731.5869-1-digetx@gmail.com
Signed-off-by: Mark Brown <broonie@kernel.org>