mfd: sta2x11: Build warning fix

This driver cannot be a module, so "remove" is never called.  The
mishap is mine, and back then there was no warning due to __devexit().

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>
Acked-by: Giancarlo Asnaghi <giancarlo.asnaghi@st.com>
Acked-by: Davide Ciminaghi <ciminaghi@gnudd.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>