UART driver: TX fifo prone to overrun

[nfeske]

When reading data bulk via UART from the modem (e.g., obtaining the list of SMS messages), I observed that sequences of bytes went missing. The first few lines of output were complete. Afterwards, the output got flaky. This observation hints at a possible FIFO overrun at the SoC side.

[nfeske]

Commit https://github.com/genodelabs/genode-allwinner/commit/574b14530219d8e9914b324a871bc9c30c17f423 fixes the problem by flushing the fifo at once instead of delivering each byte to the terminal individually.