Input: synaptics - process button bits in AGM packets

AGM packets contain valid button bits, too.
This patch refactors packet processing to parse button bits in AGM packets.
However, they aren't actually used or reported.

The point is to more completely process AGM packets,
and prepare for future patches that may actually use AGM packet button bits.

Signed-off-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>