Input: ALPS - use function pointers for different protocol handlers

In anticipation of adding more ALPS protocols and more per-device quirks,
use function pointers instead of switch statements to call functions that
differ from one device to the next.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Tested-by: Dave Turvene <dturvene@dahetral.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>