The code to set the MMC partition uses an weak function to obtain the
correct partition number.  Use that instead of the compile-time default
when deciding whether it needs to switch back.

Fixes: 6e7b7df4 ("env_mmc: support env partition setup in runtime")
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Acked-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>