iio: dac: ad5592r: localize locks only where needed in ad5592r_read_raw()

iio: dac: ad5592r: localize locks only where needed in ad5592r_read_raw()

Since there was a recently discovered issue with these locks, it probably
makes sense to cleanup the code a bit, to prevent it from being used as an
example/reference.

This change moves the lock only where it is explicitly needed to protect
resources from potential concurrent accesses.

It also reworks the switch statements to do direct returns vs caching the
return value on a variable.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20200706110259.23947-3-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>