Input: synaptics - ignore bogus mt packet

In multitouch mode, at least one device (fw: 7.4 id: 0x1c0b1) sometimes
sends a final main packet with x == 1. Since the normal values are above
1472, this is clearly bogus. At the same time, a two-finger touch is
signaled, even though only one finger was on the pad to begin with. This
patch ignores the packet altogether, removing the problem.

Acked-by: Chris Bagwell <chris@cnpbagwell.com>
Acked-by: Chase Douglas <chase.douglas@canonical.com>
Acked-by: Dmitry Torokhov <dtor@mail.ru>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>