mtd: Fix partition alignment check on multi-erasesize devices

Commit 1eeef2d7

 ("mtd: handle partitioning on devices with 0
erasesize") introduced a regression on heterogeneous erase region
devices. Alignment of the partition was tested against the master
eraseblock size which can be bigger than the slave one, thus leading
to some partitions being marked as read-only.

Update wr_alignment to match this slave erasesize after this erasesize
has been determined by picking the biggest erasesize of all the regions
embedded in the MTD partition.

Reported-by: Mathias Thore <Mathias.Thore@infinera.com>
Fixes: 1eeef2d7

 ("mtd: handle partitioning on devices with 0 erasesize")
Cc: <stable@vger.kernel.org>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Mathias Thore <Mathias.Thore@infinera.com>
Reviewed-by: Mathias Thore <Mathias.Thore@infinera.com>