Fix DSP condition during context switch

Before this patch, on a (has_fpu && !has_dsp) bitmap, the context
switch process always tries to save/restore non-existing DSP CSR,
and thus causes non-recoverable illegal instruction exception.

Author: NonerKao

arch/riscv/include/asm/switch_to.h

@@ -81,8 +81,10 @@ static inline void __switch_to_aux(struct task_struct *prev,
 	fstate_restore(next, task_pt_regs(next));
 #endif
 #ifdef CONFIG_DSP
-	dspstate_save(prev);
-	dspstate_restore(next);
+	if (has_dsp) {
+		dspstate_save(prev);
+		dspstate_restore(next);
+	}
 #endif
 }