serial: 8250: Fix THRE flag usage for CAP_MINI

The BCM2835 MINI UART has non-standard THRE semantics. Conventionally
the bit means that the FIFO is empty (although there may still be a
byte in the transmit register), but on 2835 it indicates that the FIFO
is not empty. This causes interrupts after every byte is transmitted,
with the FIFO providing some interrupt latency tolerance.

A consequence of this difference is that the usual strategy of writing
multiple bytes into the TX FIFO after checking THRE once is unsafe.
In the worst case of 7 bytes in the FIFO, writing 8 bytes loses all
but the first since by then the FIFO is full.

There is an HFIFO ("Hidden FIFO") bit which is almost what is needed,
but it only adds more bytes while both THRE and TEMT are set, i.e.
when the TX side is completely idle. This is unnecessarily pessimistic.

Add a new special case, predicated on CAP_MINI, that loops until THRE
is no longer set. With this change, the FIFO fills quickly but
subsequent writes are paced by the transmission rate.

See: https://github.com/raspberrypi/linux/issues/1855



Signed-off-by: Phil Elwell <phil@raspberrypi.org>