[drivers][serial_v2] 修复dma下rx_flush的bug #6887

Job	Run time
🔍 Summary of Git Diff Changes	40s
stm32l4	9m 12s
RT-Thread Online Packages (STM32F407 RT-Spark)	5m 42s
RTduino_Arduino Libraries (Raspberry Pico)	7m 24s
others_ft32_mm32_acm32	3m 16s
stm32_f2_f4	10m 20s
stm32_f7_g0_h7_mp15_u5_h5_wb5	7m 27s
at32_hc32_ht32	6m 49s
aarch64	1m 2s
nxp_renesas	5m 40s
mips	1m 22s
nuvoton	4m 25s
gd32_n32_apm32	4m 33s
K230	1m 27s
Infineon_TI_microchip	5m 36s
riscv-none	2m 54s
ESP32C3	7m 32s
arm-none-bsp-smart	2m 31s
i386-unknown	1m 3s
simulator	59s
stm32f0_f1	5m 45s
llvm-arm	3m 31s
hpmicro	57s
nordic(yml)	7m 9s
xuantie-smart	57s
xuantie	1m 43s
riscv64-unknown	1m 20s
collect-artifacts	32s
	1h 51m 48s

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