mmc: omap_hsmmc: remove un-initialized callbacks from platform data

these callbacks are not set, probably legacy omap 1/2 features

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Andreas Fenkart <afenkart@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>