mmc: zynq_sdhci: fix uninitialized pointer deref on probe

Since commit 3d296365 ("mmc: sdhci: Add support for
sdhci-caps-mask") sdhci_setup_cfg() expects a valid sdhci_host mmc
field. Move the mmc field initialization before sdhci_setup_cfg()
call to avoid crash on mmc pointer dereference.

[this patch is based on commit 41a9fab8 ("mmc: mv_sdhci: fix
uninitialized pointer deref on probe") by Baruch Siach]

Fixes: 3d296365 ("mmc: sdhci: Add support for sdhci-caps-mask")
Cc: Faiz Abbas <faiz_abbas@ti.com>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Matwey V. Kornilov <matwey.kornilov@gmail.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Michal Simek <michal.simek@xilinx.com> (on zcu102/zc706)