Staging: ste_rmi4: constify synaptics_rmi4_platform_data structures

This synaptics_rmi4_platform_data structure is never modified, so declare
it as const.  Others are already const.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>