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>