dfu: mtd: fix the trace when limit is reached

The offset variable = 'off' used in the error trace when limit is reach
on erase operation is incorect as 'erase_op.addr' is used in the loop.
This patch corrects the copy paste issue between the erase loop and
the write loop.

This patch also adds the 'remaining' information to allow to debug of
limit issues.

Fixes: 6015af28 ("dfu: add backend for MTD device")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>