Input: replace hard coded string with __func__ in pr_err()

Change hardcoded string "input_set_capability" in pr_err() function call,
replace it with "%s" __func__ instead.

Signed-off-by: Nick Simonov <nicksimonovv@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>