sunxi: musb: Remove unused sunxi_musb_exit method

Remove the unused sunxi_musb_exit method, there is no code in u-boot
calling the exit method, and our implementation was broken as it did
not disable the clocks and asserted reset.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>