bcm2835-mmc: Handle mmc_add_host() errors

The BCM2835 MMC host driver calls mmc_add_host() but doesn't check its
return value.  Errors occurring in that function are therefore not
handled.  Fix it.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Frank Pavlic <f.pavlic@kunbus.de>