mfd: sta2x11-mfd: Use one lock per device instead of one lock per mfd

The lock is used to implement atomic operations on each platform
device's registers, so it looks reasonable having one lock per
device instead of one common lock for all the devices belonging
to the same sta2x11 instance.

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