serial: tegra: correct help message in Kconfig from 'ttyHS' to 'ttyTHS'

ttyTHS is consistent with the name used in driver.

Signed-off-by: Richard Zhao <rizhao@nvidia.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>