Input: focaltech - mark focaltech_set_resolution() static

We get 1 warning when building kernel with W=1:
drivers/input/mouse/focaltech.c:393:6: warning: no previous prototype for 'focaltech_set_resolution' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
So this patch marks it 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>