mmc: core: Remove unnecessary retry mechanism at SDIO attach

The retry and fallback mechanism when failing to switch to 1.8V
signaling voltage is handled by the SDIO card init function. Thus we
can remove the duplicated old code from the attach function.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>