lantiq: fix patching the wifi mac address on BTHOMEHUBV3A

The firmware hotplug script tries to read the mac address from a
partition with the name "uboot-env" which does not exist (instead it's
name is uboot_env). This broke calculation of the new checksum (after
patching the mac address) which resulted in ath9k refusing to use the
EEPROM data.

The original error reported by ath9k was:
PCI: Enabling device 0000:00:0e.0 (0000 -> 0002)
ath: phy0: Bad EEPROM checksum 0x2523
ath: phy0: Unable to initialize hardware; initialization status: -22
ath9k 0000:00:0e.0: Failed to initialize device

Fixes: a2061686 ("lantiq: use ath9k device tree bindings
binding/owl-loader")

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>