staging: ste_rmi4: Make synaptics_rmi4_touchpad_config symbol static

'synaptics_rmi4_touchpad_config' is used only in this file.
Make it static. Silences the following sparse warning:
drivers/staging/ste_rmi4/synaptics_i2c_rmi4.c:652:5: warning:
symbol 'synaptics_rmi4_touchpad_config' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Naveen Kumar G <naveen.gaddipati@stericsson.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>