drivers: serial: uart_csk6: fix the bug that asysc_data is not cleared before startup

3 job 于bugfix/uart_async in 5 minute 和 39 second (queued for 1 second)
状态 作业 ID 名称 覆盖率
  Ztest
已通过 #41170
zgsz-02 generic
ztest_build_only

00:02:30

已通过 #41174
with-device zgsz-03
ztest_device_test

00:03:09

已失败 #41171
zgsz-03 with-device
ztest_device_test

00:03:43