dfu: mtd: remove direct call of mtdparts_init function

With MTD support in driver model, the direct call of mtdparts_init
should be avoided and replaced by mtd_probe_devices.

With the modificaton when MTDIDS/MTDPARTS are empty the OF fallback
with partition describe in device tree is correctly performed,
introduced by commit dc339bf7 ("mtd: add support for parsing
partitions defined in OF").
With this patch the dependency with CONFIG_CMD_MTDPARTS is removed.

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