libmtd: do not ignore non-zero eraseblock size when MTD_NO_ERASE is set

libmtd: do not ignore non-zero eraseblock size when MTD_NO_ERASE is set

In 54d68799, mtd->eb_cnt was enforced to be '1' if MTD_NO_ERASE is set.
This was done with the aim of preventing divisions by zero.

However, even if MTD_NO_ERASE is set, mtd->eb_size (eraseblock size) can
still be set to a non-zero value which would not cause a division by
zero.

Instead, enforcing an eraseblock count of '1' here even leads to
inconsistent eraseblock counting in mtd-utils and lets for example a
'flash_erase' on an mtdnand device fail:

| # flash_erase /dev/mtd0 0 0
| Erasing 32768 Kibyte @ 0 --  0 % complete libmtd: error!: bad eraseblock number 255, mtd0 has 1 eraseblocks
| flash_erase: error!: /dev/mtd0: MTD Erase entire chip failureTrying one by one each sector.
|              error 22 (Invalid argument)
| Erasing 128 Kibyte @ 0 --  0 % complete libmtd: error!: bad eraseblock number 1, mtd0 has 1 eraseblocks
| flash_erase: error!: /dev/mtd0: MTD get bad block failed
|              error 22 (Invalid argument)

Also mtdinfo would look inconsistent (eraseblock size vs amount):

| # mtdinfo /dev/mtd0
| mtd0
| Name:                           mtdram test device
| Type:                           ram
| Eraseblock size:                131072 bytes, 128.0 KiB
| Amount of eraseblocks:          1 (33554432 bytes, 32.0 MiB)
| Minimum input/output unit size: 1 byte
| Sub-page size:                  1 byte
| Character device major/minor:   90:0
| Bad blocks are allowed:         false
| Device is writable:             true

Fix this by enforcing mtd->eb_cnt to be '1' only when mtd->eb_size is
actually zero and would lead to a division by zero otherwise.

Fixes: 54d68799 ("libmtd: avoid divide by zero")
Signed-off-by: Enrico Jorns <ejo@pengutronix.de>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>