mmc: sunxi: fix timeout in sunxi_mmc_oclk_onoff

The 250ms timeout is too short.

On my system enabling the oclk takes under 50ms and disabling slightly
over 100ms when idle. Under load disabling the clock can take over
350ms.

This does not make mmc clock gating look like good option to have on
sunxi but the system should not crash with mmc clock gating enabled
nonetheless.

This patch sets the timeout to 750ms.

Signed-off-by: Michal Suchanek <hramrach@gmail.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>