mmc: sdhci: Always pass clock request value zero to set_clock host op

To allow the set_clock host op to disable the SDCLK source when not
needed, always call the host op when the requested clock speed is
zero.  Do this even if host->clock already equals zero, because
the SDHCI driver may set that value (without calling the host op)
to force an update at the next (non-zero-speed) call.

Signed-off-by: Todd Poynor <toddpoynor@google.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>