musb: proper hookup to transceiver drivers

Let the otg_transceiver in MUSB be managed by an external driver;
don't assume it's integrated.  OMAP3 chips need it to be external,
and there may be ways to interact with the transceiver which add
functionality to the system.

Platform init code is responsible for setting up the transeciver,
probably using the NOP transceiver for integrated transceivers.
External ones will use whatever the board init code provided,
such as twl4030 or something more hands-off.

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