semihosting: Adjust error checking in common_semi_cb

The err parameter is non-zero if and only if an error occured.
Use this instead of ret == -1 for determining if we need to
update the saved errno.

This fixes the errno setting of SYS_ISTTY, which returns 0 on
error, not -1.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

semihosting/arm-compat-semi.c

@@ -290,7 +290,7 @@ static target_ulong common_semi_syscall_len;
 
 static void common_semi_cb(CPUState *cs, target_ulong ret, target_ulong err)
 {
-    if (ret == (target_ulong)-1) {
+    if (err) {
 #ifdef CONFIG_USER_ONLY
         TaskState *ts = cs->opaque;
         ts->swi_errno = err;