mtd: Make sure mtd->erasesize is valid even if the partition is of size 0

Commit 33f45c44 ("mtd: Do not allow MTD devices with inconsistent
erase properties") introduced a check to make sure ->erasesize and
->_erase values are consistent with the MTD_NO_ERASE flag.
This patch did not take the 0 bytes partition case into account which
can happen when the defined partition is outside the flash device memory
range. Fix that by setting the partition erasesize to the parent
erasesize.

Fixes: 33f45c44

 ("mtd: Do not allow MTD devices with inconsistent erase properties")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: <stable@vger.kernel.org>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>