dfu: dfu_mtd: set max_buf_size to erasesize also for NOR devices

For NOR devices the logical DFU buffer size is the sector_size,
as it is done in dfu_sf.c or in spi/sf_mtd.c
(sf_mtd_info.erasesize = flash->sector_size)

For NAND the DFU size was already limited to erasesize as
has_pages = true.

So the mtd dfu backend can use this erasesize for all the MTD devices,
NOR and NAND with dfu->max_buf_size = mtd->erasesize

This difference was initially copied from MTD command, where
data is fully available in RAM without size limitation.

This patch avoids to have many sector write in dfu_mtd.c at the end
of the DFU transfer and avoids issues with USB timeout or WATCHDOG.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>