goldfish_pipe: make pipe_dev static

Make this static as it's only referenced in this source and
it does not need global scope.

Cleans up a sparse warning:

drivers/platform/goldfish/goldfish_pipe.c: warning: symbol
  'pipe_dev' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>