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