lm32: milkymist-tmu2: fix integer overflow

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

Spotted by coverity, CID 1167561.

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