mfd: twl-core: Move 'inuse' check early at probe time

We can fail earlier in case multiple instance of the twl-core is tried to
be loaded.
The twl-core by design only supports one instance.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>