usb: musb: sunxi: make unexported symbols static

The sunxi_musb_dma_controller_create and _destroy are not exported
or used outside the driver, so fix sparse warnings by making these
two static:

drivers/usb/musb/sunxi.c:357:23: warning: symbol 'sunxi_musb_dma_controller_create' was not declared. Should it be static?
drivers/usb/musb/sunxi.c:363:6: warning: symbol 'sunxi_musb_dma_controller_destroy' was not declared. Should it be static?

Signed-off-by: Ben Dooks <ben.dooks@codethink.co.uk>
Signed-off-by: Bin Liu <b-liu@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>