Input: synaptics - report multi-taps only if supported by the device

According to Section 2.4.4 of the Synaptics TouchPad Interfacing
Guide, bit 2 specifies if multi-finger detection is provided by
the touchpad. Thus, only set BTN_TOOL_DOUBLETAP and
BTN_TOOL_TRIPLETAP if the device actually supports it.

Signed-off-by: Peter Hutterer <peter.hutterer@redhat.com>
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>