firmware-utils: mktplinkfw2: support additional hardware version

firmware-utils: mktplinkfw2: support additional hardware version

As it turned out, some of new MediaTek based TP-Link devices use value
from field at 0x3c offset in version 3 of TP-Link header to specify
"Additional Hardware Version".

Value from this field is validated during regular (GUI) firmware upgrade
on devices like TL-WR840N v4 or TL-WR841N v13. If it's zero (based on
some tests, it seems that firmware will accept anything != 0), errors
like below are printed on console and upgrade fails:

[ rsl_sys_updateFirmware ] 2137:  Firmware Additional HardwareVersion
check failed

[ rdp_updateFirmware ] 345:  perror:4506

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>