milkymist-pfpu: fix potential integer overflow

Since the lm32 is a 32 bit architecture, just return a 32 bit value which
is then converted to a 64 bit value.

Spotted by coverity, CID 1005506.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
