freertos: riscv: fix t0 restore in ecall_yield

In trap entry, t0 is saved to mscratch.

Signed-off-by: wangfei_chen <wangfei_chen@realsil.com.cn>

Author: wangfei_chen

freertos/portable/GCC/RISC-V/portASM.S

@@ -529,7 +529,7 @@ ecall_end:
 
 ecall_yield:
 	/* restore t0 */
-	csrr t0, mstatus
+	csrr t0, mscratch
 	portSAVE_BaseReg
 	/* a4 = mepc
 	 * a5 = mstatus