platform: goldfish: pipe: Move the file-scope goldfish_pipe_dev variable into the driver state

This is the last patch in the series of patches to move file-scope
variables into the driver state. This change will help to introduce
another version of the pipe driver (with different state) for the
older host interface or having several instances of this device.

Signed-off-by: Roman Kiryanov <rkir@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>