spl: mmc: Unify non/driver model spl_mmc_find_device()

It is risky to have two different functions with much the same code. Future
authors may update one but not the other. It is hard to see which parts are
the same and which are different.

Unify the functions and drop the differences that are not really needed.

Note that one puts() becomes printf() as Tom mentioned that this does not
affect image size:

https://patchwork.ozlabs.org/patch/537276/



Note: It would be better to have an empty printf() and avoid the #ifdef for
CONFIG_SPL_LIBCOMMON_SUPPORT.

Signed-off-by: Simon Glass <sjg@chromium.org>
Tested-by: Michal Simek <michal.simek@xilinx.com>