mfd: Remove unused variable from da9052_device_init

This is to address the following warning during compilation time:

  drivers/mfd/da9052-core.c: In function ‘da9052_device_init’:
  drivers/mfd/da9052-core.c:646: warning: unused variable ‘desc’

This variable is indeed no longer in use (change can be traced back
to commit: 86144194).
Signed-off-by: Krzysztof Wilczynski <krzysztof.wilczynski@linux.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/da9052-core.c

@@ -772,7 +772,6 @@ EXPORT_SYMBOL_GPL(da9052_regmap_config);
 int __devinit da9052_device_init(struct da9052 *da9052, u8 chip_id)
 {
 	struct da9052_pdata *pdata = da9052->dev->platform_data;
-	struct irq_desc *desc;
 	int ret;
 
 	mutex_init(&da9052->auxadc_lock);