pinctrl: intel: Simplify offset validation in intel_get_padcfg()

There is more generic and simpler validation just against the nregs.
Using it allows to drop customization from the intel_get_padcfg().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>