usb: musb: sunxi: Don't print error on MUSB_ULPI_BUSCONTROL access

Error message appears during suspend, where musb driver is storing
the register state in musb_save_context():
```
musb-sunxi 1c19000.usb: Error unknown readb offset 112
```

Print warning instead to avoid confusion.

Signed-off-by: Roman Stratiienko <r.stratiienko@gmail.com>
Link: https://lore.kernel.org/r/20211019131244.1568560-1-r.stratiienko@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>