mfd: Rename mfd_shared_cell_{en,dis}able to drop the "shared" part

As requested by Samuel, there's not really any reason to have "shared"
in the name.

This also modifies the only user of the function, as well.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>