mmc: mv_sdhci: zero out sdhci_host structure

The mv_sdhci driver was not zeroing the sdhci_host structure it
allocates causing random access violations in parts of the mmc core
where the "ops" member pointers are checked and called if not NULL.

Signed-off-by: Matt Pelland <mpelland@starry.com>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>