input: synaptics_i2c_rmi4: silence compiler warnings

Many functions in the driver are called only from PM suspend/resume
callbacks. When CONFIG_PM is not enabled in defconfig, compiler
warns about these functions being defined but unused. While a #ifdef
can be added around the function declaration to avoid this warning,
a clean solution is to mark it as  __maybe_unused, which will let
gcc silently drop the function definition when it is not needed.
Prototypes of these functions are marked with '__maybe_unused'
annotation in order to remove this warning. Also remove dead
code from the driver.

Change-Id: I29b9f3e687fb68dc7e75824017f21a570a90e703
Signed-off-by: Himanshu Aggarwal <haggarwa@codeaurora.org>