mfd: sta2x11-mfd: Only add sta2x11_mfd if it hasn't already been added

The pci probe method is called twice now, so we have to call
sta2x11_mfd_add() only once to avoid a -EBUSY error.

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