mmc: tegra: priv struct and naming cleanup

struct mmc_host is a Tegra-specific structure, but the name implies it's
something defined by core MMC code, which is confusing. Rename it to
struct tegra_mmc_priv to make its purpose more obvious. The new name is
also more appropriate for a DM driver private data structure, which will
be relevant later in this series.

Nothing needs access to this type except the MMC driver itself. Move the
definition into the driver C file.

Make sure all Tegra MMC functions are named tegra_mmc_*. Even though
they're all static, it's useful to have good naming so that symbol tables
are easy to interpret. A few functions aren't renamed by this patch since
they'll be deleted by a subsequent patch in this series.

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