input: synaptics_dsx: allocate heap memory for temp buf

rmidev file operations structure include write() and
read() which accepts data from user space. Temp
buffers are allocated through variable length arrays
which can pose security problems. So allocate memory
on heap instead of stack to avoid this.

Bug: 28799389
Bug: 31554728
CRs-Fixed: 1032459
Change-Id: I44443f91d435715dd0097ef8e8dfc48e291f93fc
Tracked-On: https://jira01.devtools.intel.com/browse/AW-1795


Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>
Signed-off-by: Biswajit Paul <biswajitpaul@codeaurora.org>
Signed-off-by: Victor Tasayco Loarte <victorx.tasayco.loarte@intel.com>