For 64-bit implementations, the mullw result is the 64 bit product
of the sign-extended least significant 32 bits of the source
registers.

Fix the code to properly sign extend the source operands and produce
a 64 bit product.

Example:
R3 00000000002F37A0
R4 41C33D242F816715
mullw 3,3,4
R3 expected : 0008C3146AE0F020
R3 actual   : 000000006AE0F020 (without this patch)
Signed-off-by: NTom Musta <tommusta@gmail.com>
Signed-off-by: NAlexander Graf <agraf@suse.de>