mmc: sunxi: Reset the device at probe time

We might be into some troubles if the bootloader misconfigured the MMC
controller.

We currently only de-assert the reset line at probe time, which means that
if the device was already out of reset, we're going to keep whatever state
was set already.

Switch to a reset instead of the deassert to have a device in a pristine
state when we start operating.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>