mfd: ab8500-gpadc: Suppress 'ignoring regulator_enable() return value' warning

drivers/mfd/ab8500-gpadc.c: In function ‘ab8500_gpadc_resume’:
drivers/mfd/ab8500-gpadc.c:911:18: warning: ignoring return value of
        ‘regulator_enable’, declared with attribute warn_unused_result
        [-Wunused-result]

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Srinidhi Kasagar <srinidhi.kasagar@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>