mmc: tegra: don't use periph_id in pad_init_mmc()

The MMC driver will soon be converted to use standard clock/reset APIs,
and so the periph_id field in the MMC device priv struct will disappear.
Rework the implementation of pad_init_mmc() to rely on this; using the
device register address is a much more direct test anyway.

Cc: Jaehoon Chung <jh80.chung@samsung.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>