Revert "kernel: mtd: Make subpartitions inherit parent's access mode"

Revert "kernel: mtd: Make subpartitions inherit parent's access mode"

This reverts commit de80424f706682e8bba27c60bcd2a9c1b4a5e875 which needs
more work and testing as it broke at least jffs2 overlays at least on
ath79 platform, marking them as read-only, thus unusable:

 jffs2_build_filesystem(): erasing all blocks after the end marker...
 jffs2: Erase at 0x009e0000 failed immediately: -EROFS. Is the sector locked?

Ref: http://lists.infradead.org/pipermail/openwrt-devel/2020-January/021344.html
Reported-by: Steve Brown <sbrown@ewol.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>