omap_hsmmc: Add missing probe handler hook to platform driver data

The missing probe handler hook will never probe the driver. Add it back.
Fixes broken MMC on OMAP.

We use platform_driver_probe() API since omap_hsmmc is not a hot-pluggable
device.

Signed-off-by: Roger Quadros <ext-roger.quadros@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>