xtensa: fix syscall_set_return_value

syscall return value is in the register a2, not a0.

Cc: stable@vger.kernel.org # v5.0+
Fixes: 9f24f3c1 ("xtensa: implement tracehook functions and enable HAVE_ARCH_TRACEHOOK")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/include/asm/syscall.h

@@ -51,7 +51,7 @@ static inline void syscall_set_return_value(struct task_struct *task,
 					    struct pt_regs *regs,
 					    int error, long val)
 {
-	regs->areg[0] = (long) error ? error : val;
+	regs->areg[2] = (long) error ? error : val;
 }
 
 #define SYSCALL_MAX_ARGS 6