sunxi: Remove misleading USB-OTG charger message

The sunxi MUSB glue driver has some code to check for external VBUS
presence when it's going to use the MUSB host mode, and it warns if
there is VBUS provided through the cable (in sunxi_musb_enable()).

This code was apparently copied to the USB gadget detection code
(g_dnl_board_usb_cable_connected()), but here we actually *expect*
external VBUS power, so a warning is wrong and confusing.
So far this message rarely triggered, but a recent patch (6fa41cdd)
changed this:
===========================
=> ums 0 mmc 2
UMS: LUN 0, dev mmc 2, hwpart 0, sector 0x0, count 0xe90000
A charger is plugged into the OTG
/A charger is plugged into the OTG
\A charger is plugged into the OTG
|A charger is plugged into the OTG
-A charger is plugged into the OTG
....
===========================

Remove the message for the gadget cable detection call, and just return
the status of the VBUS detection, as this is what the callers are after.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>