iio:addac:ad74413r: Fix uninitialized ret in a path that won't be hit.

I don't believe it's possible to hit this, because we drop
out of __iio_update_buffers() earlier in the event of an empty
list.  However, that is not visible to the compiler so lets
return an error if we do hit the loop with an empty bitmask.

Fixes: 5d97d9e9 ("iio: addac: ad74413r: fix off by one in ad74413r_parse_channel_config()")
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Cc: Cosmin Tanislav <cosmin.tanislav@analog.com>
Link: https://lore.kernel.org/r/20211220164726.3136307-1-jic23@kernel.org