i2c: tegra: change phrasing, "fallbacking" to "falling back"

The phrasing in two dev_err messages is using fallbacking which
os less understandable than "falling back", so fix this up.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>