USB: serial: keyspan_pda: add write-fifo support

Use the port write fifo and generic chars_and_buffer and write_room
implementations when writing. This not only allows for more efficient
transfers, but more importantly fixes the remaining issues related to
the conservative write_room() implementation which could prevent the
line discipline from making forward progress (e.g. waiting for n > 1
bytes of space to become available).

Note that this also allows using the driver for the system console
without dropping data when the write URB is busy (including when adding
carriage return on line feed).

Acked-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Johan Hovold <johan@kernel.org>