sparc64: repair calling incorrect hweight function from stubs

Commit v4.12-rc4-1-g9289ea7f952b introduced a mistake that made the
64-bit hweight stub call the 16-bit hweight function.

Fixes: 9289ea7f

 ("sparc64: Use indirect calls in hamming weight stubs")
Signed-off-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: David S. Miller <davem@davemloft.net>