Input: wistron_btns - support Prestigio Wifi RF kill button

The Prestigio 157, an old no-name clone laptop uses input keys very
similar to the Wistron 1557/MS2141 with the addition of BIOS-controlled
wireless radio frequency kill switch.

This patch adds support for the RF kill switch control and adds manual
identification of the model.

The Prestigio does not expose any recognisable identity via dmidecode
and so requires manual selection at module init using

force=1 keymap=prestigio

Signed-off-by: TJ <ubuntu@tjworld.net>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>