uart8250: Add tx_empty() timeout
uart8250: Add tx_empty() timeout

Unfortunately, certain hardware like VirtualBox's 16550 doesn't ever
drain TX, ever. So add a timeout, as to not hang the whole OS.

Signed-off-by: Pedro Falcato <d037c3eb54456eb57a351d3e1361e11b3d89c857@gmail.com>

heatd/Onyx,heatd/Onyx,heatd/Onyx,heatd/Onyx