Input: aiptek - kill aiptek_convert_from_2s_complement()

There is no reason to do that, just tell the compiler that
we are dealing with signed values in buffer, that's it.

Signed-off-by: Rene van Paassen <rene.vanpaassen@gmail.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>