powerpc: Fix casueword(9) post-r349951

'=' asm constraint marks a variable as write-only. Because of this, gcc
throws away the initialization of 'res', causing garbage to be returned if
the CAS was successful. Use '+' to mark res as read/write, so that the
initialization stays in the generated asm. Also, fix the reservation
clearing stwcx store index register in casueword32, and only do the dummy
store when needed, skip it if the real store has already succeeded.