drivers: serial: bt: Make TX flush non-deferrable

To prevent postponing data flush until filling fifo. Now the flush will
occur at the scheduled time, regardless of subsequent poll_out reqs.

Signed-off-by: Luis Ubieda <luisf@croxel.com>

drivers/serial/uart_bt.c

@@ -169,7 +169,7 @@ static void uart_bt_poll_out(const struct device *dev, unsigned char c)
 		 * data, so more than one byte is transmitted (e.g: when poll_out is
 		 * called inside a for-loop).
 		 */
-		k_work_reschedule(&dev_data->uart.tx_work, K_MSEC(1));
+		k_work_schedule(&dev_data->uart.tx_work, K_MSEC(1));
 	}
 }