usb: musb: correct Kconfig in order to avoid non compilable selection

Currently it is possible to have:

USB_MUSB_OMAP2PLUS=m
TWL4030_USB=y

which would result compile time error due to missing symbols.

With this change USB_MUSB_OMAP2PLUS and TWL4030_USB will be in sync.

Reported-by: Vincent Stehle <v-stehle@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>