uart: fix uart_tx_wait_idle to wait for fifo empty

uart: fix uart_tx_wait_idle to wait for fifo empty

In some cases, when data was just written into UART FIFO, transmitter
state could be still zero while the FIFO did contain some data.
This resulted in uart_tx_wait_idle occasionally returning before all
the data was sent out.
Fix by checking both UART transmitter state and TX FIFO count.