Input: tegra-kbc - fix computation of polling time

Fix a constant definition and computation of polling time.

[dtor@mail.ru: switched to using DIV_ROUND_UP as was suggested by
 Thierry Reding <thierry.reding@avionic-design.de>]

Signed-off-by: Rakesh Iyer <riyer@nvidia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>