tty: amba-pl011: Avoid rare write-when-full error

Under some circumstances on BCM283x processors data loss can be
observed - a single byte missing from the TX output stream. These bytes
are always the last byte of a batch of 8 written from pl011_tx_chars
when from_irq is true, meaning that the FIFO full flag is not checked
before writing.

The transmit optimisation relies on the FIFO being half-empty when the
TX interrupt is raised. Instrumenting the driver further showed that
the failure case correlated with the TX FIFO full flag being set at the
point where the last byte was written to the data register, which
explains the data loss but not how the FIFO appeared to be prematurely
full. A possible explanation is that a FIFO write was in flight at the
time the interrupt was raised, but as yet there is no hypothesis as to
how this might occur.

In the absence of a clear understanding of the failure mechanism, avoid
the problem by checking the FIFO levels before writing the last byte of
the group, which will have minimal performance impact.

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