target/openrisc: Do not use target_ulong for @mr in MTSPR helper

OpenRISCTLBEntry::@mr field is a uint32_t type since its
introduction in commit 726fe04572 ("target-or32: Add MMU support").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Anton Johansson <anjo@rev.ng>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20251010070702.51484-3-philmd@linaro.org>

target/openrisc/sys_helper.c

@@ -45,7 +45,7 @@ void HELPER(mtspr)(CPUOpenRISCState *env, target_ulong spr, target_ulong rb)
     OpenRISCCPU *cpu = env_archcpu(env);
 #ifndef CONFIG_USER_ONLY
     CPUState *cs = env_cpu(env);
-    target_ulong mr;
+    uint32_t mr;
     int idx;
 #endif