spl: mmc: refactor device location code to its own function

spl: mmc: refactor device location code to its own function

Simplify spl_mmc_load_image() code by moving the part that finds the mmc device
into its own function spl_mmc_find_device(), available in two flavors: DM and
non-DM.

This refactor fixes a bug in which an error in the device location sequence
does not necessarily aborts the rest of the code. With this refactor, we fail
the moment there is an error.

Signed-off-by: Nikita Kiryanov <nikita@compulab.co.il>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Paul Kocialkowski <contact@paulk.fr>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>