lib/mpi: fix build with clang

Use just @ to denote comments which works with gcc and clang.
Otherwise clang reports an escape sequence error:
  error: invalid % escape in inline assembly string

Use %0-%3 as operand references, this avoids:
  error: invalid operand in inline asm: 'umull ${1:r}, ${0:r}, ${2:r}, ${3:r}'

Also remove superfluous casts on output operands to avoid warnings
such as:
  warning: invalid use of a cast in an inline asm context requiring an l-value

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>