lm32: milkymist-tmu2: fix another 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 commit 237a8650,
in a similar way, and is the other half of the fix for
coverity CID 1167561.

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