usb: musb: core: add common method of getting vbus status

Fix musb_platform_get_vbus_status return value in case of platform
implementation is not defined, bringing expected behaviour of
musb_platform_get wrapper. Add musb_vbus_show default method to determine
VBUS status in case platform method is not defined.

Signed-off-by: Roman Alyautdin <ralyautdin@dev.rtsoft.ru>
Signed-off-by: Felipe Balbi <balbi@ti.com>