staging: iio: ad5933: Return correct value for AD5933_OUT_RANGE.

The 'break' statement after calling ad5933_cmd only breaks out of the
'for' loop, which then unconditionally sets the return value to -EINVAL.
Move the initialisation of 'ret' so we return the correct value.

Signed-off-by: Phil Turnbull <phil.turnbull@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>