mmc: tegra: support 4-bit operation too on 8-bit slots

If a board has all 8 data lines routed, the SD/MMC controller can still
operate in 4-bit (or presumably even 1-bit) mode. Adjust Tegra's MMC
driver to report the 4-bit capability even for 8-bit slots.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>