dmm/eev121gw: silence compiler warning (unused value)

Silence a compiler warning about not using an assigned value in later
instructions. This is about the EEVBlog 121GW meter's date and device
number in received serial packets, which are worth keeping around for
future versions of the driver.

../src/dmm/eev121gw.c:728:2: warning: Value stored to 'raw_serial' is never read
        raw_serial = RB32(&buf[OFF_SERIAL_3]);