mmc: host: omap_hsmmc: remove unused platform callbacks

Remove unused callbacks in the omap_hsmmc_platform_data structure

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>