drivers/serial: fix race condition in multi-thread write

if multiple threads are doing serial read/write at the same time,
the driver will only wake up one of the thread, which will cause
other threads fail to be woken up in time and cause blocking

Signed-off-by: chao an <anchao@xiaomi.com>

include/nuttx/serial/serial.h

@@ -272,8 +272,6 @@ struct uart_dev_s
 
   uint8_t              open_count;   /* Number of times the device has been opened */
   uint8_t              escape;       /* Number of the character to be escaped */
-  volatile bool        xmitwaiting;  /* true: User waiting for space in xmit.buffer */
-  volatile bool        recvwaiting;  /* true: User waiting for data in recv.buffer */
 #ifdef CONFIG_SERIAL_REMOVABLE
   volatile bool        disconnected; /* true: Removable device is not connected */
 #endif