mmc: sunxi: Correcting SDXC_HARDWARE_RESET bit

Fixing the register name in sunxi_mmc_reset_host since the
SDXC_HARDWARE_RESET bit is actually located within REG_GCTRL and not
REG_CMDR as it was pointed out by Allwinner.

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>