ath79: uboot-envtools: fix partition for ZTE MF286

ath79: uboot-envtools: fix partition for ZTE MF286

By mistake, a wrong partition for U-boot environment was introduced for
ZTE MF286 while adding support, when flash layout wasn't finalized. Fix
that, according to the actual flash layout:
dev:    size   erasesize  name
mtd0: 00140000 00020000 "fota-flag"
mtd1: 00140000 00020000 "caldata"
mtd2: 00140000 00020000 "mac"
mtd3: 00f40000 00020000 "ubiconcat0"
mtd4: 00400000 00020000 "kernel"
mtd5: 06900000 00020000 "ubiconcat1"
mtd6: 00080000 00010000 "u-boot"
mtd7: 00020000 00010000 "u-boot-env"
mtd8: 07840000 00020000 "ubi"

Fixes: 8c78a13bfc1f ("ath79: support ZTE MF286")
Signed-off-by: Lech Perczak <lech.perczak@gmail.com>