mmc: mv_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.

Fixes: 3d296365 ("mmc: sdhci: Add support for sdhci-caps-mask")
Cc: Faiz Abbas <faiz_abbas@ti.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>