lm32: milkymist-tmu2: fix a third integer overflow

Don't truncate the multiplication and do a 64 bit one instead
because the result is stored in a 64 bit variable.

This fixes a similar coverity warning to commits 237a8650 and
4382fa65, in a similar way, and is the final third of the fix for
coverity CID 1167561 (hopefully!).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Acked-by: Michael Walle <michael@walle.cc>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>