mmc: omap: make it behave well as a module

Use proper __devinit and __devexit annotation for driver functions.
Instantiate the probe function for driver_ops instead of a probe in
the register function.

Signed-off-by: Venkatraman S <svenkatr@ti.com>
Acked-by: Balaji T K <balajitk@ti.com>
Signed-off-by: Chris Ball <cjb@laptop.org>