mmc: core: fix regression: set default clock gating delay to 0

A recent commit "mmc: core: Use delayed work in clock gating framework"
(597dd9d7) introduced a default 200ms delay before clock gating
actually takes place.  This means that every time an MMC interface
becomes idle it first stays on for 200ms before gating its clock. This
leads to increased power consumption and is therefore a clear regression.
This patch restores the original behaviour by setting the default delay
to 0. Users prioritising throughput over power efficiency can still
modify the delay via sysfs.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>