input: synaptics_dsx_2.6: Add check for stylus device

If stylus_enable is true and sytlus device is NULL,
it is resulting in crash while reporting input events. So,
check for existence of stylus device as well along with
stylus_enable.

Change-Id: I6679ff03f7cd4a8e587d0f4e8f794141b2cddd45
Signed-off-by: Himanshu Agrawal <hagraw@codeaurora.org>
Signed-off-by: Manjunatha Madana <mamanj@codeaurora.org>