mfd: axp20x: Drop wrong AXP288_PMIC_ADC_EN define

The adc-enable register for the axp288 is 0x82, not 0x84.
0x82 is already defined as AXP20X_ADC_EN1 and that is what the
axp288_adc driver is actually using, so simply drop the wrong
AXP288_PMIC_ADC_EN define.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>