mfd: da9052: Fix manual ADC read after timed out read

It is possible that under heavy system load, the counter in the completion
struct, used for waiting for end of AD conversion, gets incremented twice.
To make sure the driver recovers from this situation, the completion struct
should be reinitialized.

Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Lee Jones <lee.jones@linaro.org>