stm32mp: stm32prog: accept device without partition

When partitions are not available on a device the command stm32prog raises
an error but a device can have no partition to check in init_device()
and the command need to continue to the next part_id.

This patch correct an issue for ram0 target, when block_dev and mtd
are NULL.

For example with the simple flashlayout file:

Opt	Part	Name	Type	Device	Offset	Binary
-	0x01	fsbl	Binary	none	0x0	tf-a-serialboot.stm32
-	0x03	ssbl	Binary	none	0x0	u-boot.stm32
P	0x10	kernel	System	ram0	0xC2000000	uImage.bin
P	0x11	dtb	FileSytem	ram0	0xC4000000	stm32mp157f-ev1.dtb

Fixes: ffc405e6 ("stm32mp: stm32prog: add upport of partial update")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>