Input: keyboard - staticise non-exported symbols

These symbols are not used outside it's driver so no need to
make the symbol global.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>