stm32mp: fix board_get_alt_info_mmc()

MAX_SEARCH_PARTITIONS is the highest possible partition number.
Do not skip the last partition in board_get_alt_info_mmc().

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>