Input: tsc2005 - hide selftest attribute if we can't reset

If implementation to perform self-test/reset has not been provided by the
platform code hide 'selftest' sysfs attribute instead of returning error
when someone tries to use it.

Tested-by: Aaro Koskinen <aaro.koskinen@nokia.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>