mmc: atmel_sdhci: fix uninitialized pointer deref on probe

Commit 3d296365 ("mmc: sdhci: Add support for
sdhci-caps-mask") sdhci_setup_cfg() expects a valid sdhci_host mmc
field.

Fixes: 3d296365 ("mmc: sdhci: Add support for sdhci-caps-mask")
Cc: Faiz Abbas <faiz_abbas@ti.com>
Cc: Wenyou Yang <wenyou.yang@atmel.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Tested-by: Eugen Hristev <eugen.hristev@microchip.com>