mmc: sunxi: Lock fix

1) Adding a comment in order to clarify the choice of the locks within
sunxi_mmc_handle_manual_stop

2) As <lixiang@allwinnertech.com> has pointed out the wait_dma variable
was not accessed within the spin lock block in sunxi_mmc_request and so
(even if it should never happend) it would have theoretically been
possible that some other function would access the variable at the same
time as the function. This has been changed now and the function is
using local variables outside the lock and copys the value over during
the lock phase.

Signed-off-by: David Lanzendörfer <david.lanzendoerfer@o2s.ch>
Reported-by:  <lixiang@allwinnertech.com>
Acked-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>