lib/mpi: Fix building for powerpc with clang

0day reports over and over on an powerpc randconfig with clang:

lib/mpi/generic_mpih-mul1.c:37:13: error: invalid use of a cast in a
inline asm context requiring an l-value: remove the cast or build with
-fheinous-gnu-extensions

Remove the superfluous casts, which have been done previously for x86
and arm32 in commit dea632ca ("lib/mpi: fix build with clang") and
commit 7b7c1df2

 ("lib/mpi/longlong.h: fix building with 32-bit
x86").

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/ClangBuiltLinux/linux/issues/991
Link: https://lore.kernel.org/r/20200413195041.24064-1-natechancellor@gmail.com