mmc: bcm: 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: Matthias Brugger <mbrugger@suse.com>
Cc: Thomas Fitzsimmons <fitzsim@fitzsim.org>
Signed-off-by: Peng Fan <peng.fan@nxp.com>