input: synaptics_i2c_rmi4: synchronize access to support_fn_list

Synchronize access to support_fn_list to avoid race conditions.

This patch is propagated from msm-3.10 kernel
(commit: bda3d93d0c5b793c30dcfb2d17573f98e3d21973
input: synaptics_i2c_rmi4: synchronize access to support_fn_list)
Also cleaned checkpatch warning on msm-3.18 kernel.

Change-Id: I13b8e99f0c4914046c745b0232a9ce3f792bba47
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>
Signed-off-by: Sudhakar Manapati <smanap@codeaurora.org>