USB: nop-usb-xceiv: behave when linked as a module

The NOP OTG transceiver driver needs to be usable from modules.
Make sure its symbols are always accessible at both compile and
link time, and make sure the device instance is allocated from
the heap so that device lifetime rules are obeyed.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>