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>

boards/z80/z80/z80sim/src/z80_serial.c

@@ -90,8 +90,6 @@ static char g_uarttxbuffer[CONFIG_UART_TXBUFSIZE];
 static uart_dev_t g_uartport =
 {
   0,                        /* open_count */
-  false,                    /* xmitwaiting */
-  false,                    /* recvwaiting */
   true,                     /* isconsole */
   { 1 },                    /* closesem */
   { 0 },                    /* xmitsem */