input: synaptics_dsx_2.6: Propagating security fixes from msm-3.18

This commit is a squash of below security fixes from the
branch msm-3.18.

fc64cb5 touchscreen: synaptics_dsx_2.6: Remove fw_update sysfs entries
7aa4783 input: synaptics_dsx: reallocate buffer under lock.

Instead of using #ifdefs inside function, add conditionally
compiled functions.

Change-Id: I50f9800a27d7142779298d2ffa880df15b164353
Signed-off-by: Venkata Prahlad Valluru <vvalluru@codeaurora.org>