mfd: da9052-core: Use regmap_irq_get_virq() and fix the probe

mfd: da9052-core: Use regmap_irq_get_virq() and fix the probe

On a mx53qsb dt-kernel the da9052-core driver fails to probe as follows:

da9052 1-0048: DA9052 ADC IRQ failed ret=-22

The reason for the error was due to passing only the offset as the interrupt
number in request_threaded_irq() without da9052->irq_base.

The recommended approach though is to use regmap_get_virq() to acquire the
interrupt number and this allows to get rid of da9052->irq_base.

Fix it and allow the driver to probe successfully.

Also provide a few more error logs and change the irq string to "adc-irq", so
that it appears as a single word in 'cat /proc/interrupts'

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>