Every SPI transfer could have a different clock rate.  The
spi-geni-qcom controller code to deal with this was never very well
optimized and has always had a lot of code plus some calls into the
clk framework which, at the very least, would grab a mutex.  However,
until recently, the overhead wasn't _too_ much.  That changed with
commit 0e3b8a81 ("spi: spi-geni-qcom: Add interconnect support")
we're now calling geni_icc_set_bw(), which leads to a bunch of math
plus:
  geni_icc_set_bw()
    icc_set_bw()
      apply_constraints()
        qcom_icc_set()
          qcom_icc_bcm_voter_commit()
            rpmh_invalidate()
            rpmh_write_batch()
...and those rpmh commands can be a bit beefy if you call them too
often.

We already know what speed we were running at before, so if we see
that nothing has changed let's avoid the whole pile of code.

On my hardware, this made spi_geni_prepare_message() drop down from
~145 us down to ~14 us.

NOTE: Potentially it might also make sense to add some code into the
interconnect framework to avoid executing so much code when bandwidth
isn't changing, but even if we did that we still want to short circuit
here to save the extra math / clock calls.
Acked-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Akash Asthana<akashast@codeaurora.org>
Fixes: 0e3b8a81 ("spi: spi-geni-qcom: Add interconnect support")
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20200701174506.1.Icfdcee14649fc0a6c38e87477b28523d4e60bab3@changeidSigned-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>