greybus: uart: Use a fifo to send data to the modules

The firmware now buffers data instead of blocking while the transfer
is sent, and the write operation cannot sleep.

Instead of using gb_transfer_sync (which sleeps) in the write callback,
buffer data in a fifo and send it from from a work queue.

The write_room callback will will report 1 byte less that what is really
available in the fifo, to leave space for extra characters that may
be added by the tty layer.

Signed-off-by: Axel Haslam <ahaslam@baylibre.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>