cmd: mtd: solve bad block support in erase command

This patch modify the loop in mtd erase command to erase one by one
the blocks in the requested area.

It solves issue on "mtd erase" command on nand with existing bad block,
the command is interrupted on the first bad block with the trace:
	"Skipping bad block at 0xffffffffffffffff"

In MTD driver (nand/raw), when a bad block is present on the MTD
device, the erase_op.fail_addr is not updated and we have the initial
value MTD_FAIL_ADDR_UNKNOWN = (ULL)-1.

This case seems normal in nand_base.c:nand_erase_nand(),
we have the 2 exit cases during the loop:

1/ we have a bad block (nand_block_checkbad)
	instr->state = MTD_ERASE_FAILED
	loop interrupted (goto erase_exit)

2/ if block erase failed (status & NAND_STATUS_FAIL)
	instr->state = MTD_ERASE_FAILED;
	instr->fail_addr =
				((loff_t)page << chip->page_shift);
	loop interrupted (goto erase_exit)

So erase_op.fail_addr can't be used if bad blocks were present
in the erased area; we need to use mtd_erase only one block to detect
and skip these existing bad blocks (as it is done in nand_util.c).

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com>