mmc: sdhci: clarify the get_timeout_clock callback

Currently the get_timeout_clock callback doesn't clearly
have a statement that it needs the variant drivers to return
the timeout clock rate in kHz if the SDHCI_TIMEOUT_CLK_UNIT
isn't present, otherwise the variant drivers should return it
in MHz. It's also very likely that further variant drivers which
are going to use this callback will be confused by this situation.
Given the fact that moderm sdhci variant hosts are very prone to get
the timeout clock from common clock framework (actually the only three
users did that), it's more natural to return the value in Hz and we
make an explicit comment there. Then we put the unit conversion inside
the sdhci core. Thus will improve the code and prevent further misuses.

Reported-by: Anssi Hannula <anssi.hannula@bitwise.fi>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>