If you load to 81000000 the LZMA fails because the uncompressed image is large enough to overwrite the compressed data before it is finished. So load to 83000000 and boot there.
tftpboot 83000000 <filename>
bootm 83000000
OpenWrt should boot. Then scp the sysupgrade file to /tmp and use sysupgrade to install it.
Do not issue any mtd or flash commands in the bootloader. That is unnecessary and dangerous.