mfd: ab8500-gpadc: Complain if we fail to enable vtvout LDO

Since commit c8801a8e
"regulator: core: Mark all get and enable calls as __must_check",
we must check return value of regulator_enable() to silence below build warning.

  CC      drivers/mfd/ab8500-gpadc.o
drivers/mfd/ab8500-gpadc.c: In function 'ab8500_gpadc_runtime_resume':
drivers/mfd/ab8500-gpadc.c:598:18: warning: ignoring return value of 'regulator_enable', declared with attribute warn_unused_result [-Wunused-result]
drivers/mfd/ab8500-gpadc.c: In function 'ab8500_gpadc_probe':
drivers/mfd/ab8500-gpadc.c:655:18: warning: ignoring return value of 'regulator_enable', declared with attribute warn_unused_result [-Wunused-result]

Also convert to devm_regulator_get(), this fixes a missing regulator_put() call
in ab8500_gpadc_remove().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>