target-mips: Fix warning from Sparse

Sparse report:

target-mips/dsp_helper.c:3681:5: warning: returning void-valued expression

Cc: Aurelien Jarno <aurelien@aurel32.net>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

target-mips/dsp_helper.c

@@ -3678,7 +3678,7 @@ void cpu_wrdsp(uint32_t rs, uint32_t mask_num, CPUMIPSState *env)
 
 void helper_wrdsp(target_ulong rs, target_ulong mask_num, CPUMIPSState *env)
 {
-    return cpu_wrdsp(rs, mask_num, env);
+    cpu_wrdsp(rs, mask_num, env);
 }
 
 uint32_t cpu_rddsp(uint32_t mask_num, CPUMIPSState *env)