mmc: Fix uninitialised priv member

When using omap_hsmmc without the device model then the allocation
of mmc->priv ends up uninitialised.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Tested-by: Robert Nelson <robertcnelson@gmail.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>