mfd: rn5t618: Mark ADC control register volatile

commit 2f3dc25c upstream.

There is a bit which gets cleared after conversion.

Fixes: 9bb9e29c ("mfd: Add Ricoh RN5T618 PMIC core driver")
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>