mmc: bcm2835_sdhci: Use calloc to allocate bcm2835_sdhci_host

We need to clear the allocated memory explicitly as the included
struct sdhci_host has function pointers. Those are compared to NULL to
test if this (optional) feature is supported. Leaving them undefined let
u-boot jump to arbitrary memory.

Signed-off-by: Alexander Stein <alexanders83@web.de>