usb: otg: Fix build errors if USB_MUSB_OMAP2PLUS is selected as module

usb: otg: Fix build errors if USB_MUSB_OMAP2PLUS is selected as module

TWL4030_USB & TWL6030_USB must depend on USB_MUSB_OMAP2PLUS in Kconfig else
we get build errors with

CONFIG_USB_MUSB_HDRC=m
CONFIG_USB_MUSB_OMAP2PLUS=m
CONFIG_TWL4030_USB=y
CONFIG_TWL6030_USB=y

LD      init/built-in.o
drivers/built-in.o: In function `twl4030_usb_irq':
drivers/usb/otg/twl4030-usb.c:518: undefined reference to `omap_musb_mailbox'
drivers/built-in.o: In function `twl4030_usb_phy_init':
drivers/usb/otg/twl4030-usb.c:540: undefined reference to `omap_musb_mailbox'

drivers/built-in.o: In function `twl6030_usb_irq':
drivers/usb/otg/twl6030-usb.c:230: undefined reference to `omap_musb_mailbox'
drivers/usb/otg/twl6030-usb.c:225: undefined reference to `omap_musb_mailbox'
drivers/built-in.o: In function `twl6030_usbotg_irq':
drivers/usb/otg/twl6030-usb.c:259: undefined reference to `omap_musb_mailbox'

CC: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>