mmc: sh_mmcif: simplify and use meaningful label names in error-handling

A check for NULL platform data can be conveniently made in the very
beginning of probing. Replace numbered error-handling labels in .probe()
with meaningful names to make any future reorganisation simpler.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Reviewed-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Chris Ball <cjb@laptop.org>