serial: pl01x: Modify pending callback to test if transmit FIFO is empty

serial: pl01x: Modify pending callback to test if transmit FIFO is empty

Before this change the FR_TXFF (Transmit FIFO full) bit (5 in
HW_UARTDBG_FR) has been used to assess if there is still data pending
to be sent via UART.

This approach is problematic, as it may happen that serial is in the
middle of transmission (so the TX FIFO is NOT full anymore) and this
test returns true infinitely. As a result, for example in _serial_flush()
DM serial function we are locked in endless while().

The fix here is to test explicitly if the TX FIFO is empty.

Signed-off-by: Lukasz Majewski <lukma@denx.de>