usb: musb: allow building USB_MUSB_TUSB6010 as a module

Commit 1376d92f

 "usb: musb: allow musb and glue layers to be modules"
made the USB_MUSB_TUSB6010 option modular, but actually building
the driver as a module does not work, so various randconfig builds
actually fail. This changes all code that depends on the
option to also check for modular builds, and exports the necessary
symbols.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>