iio: proximity: srf08: use stack allocated scan buffer

iio: proximity: srf08: use stack allocated scan buffer

Use a stack allocated scan struct in srf08_trigger_handler(). Since the
scan buffer isn't used outside of this function and doesn't need to be
DMA-safe, it doesn't need to be in struct srf08_data. We can also
eliminate an extra local variable for the return value of
srf08_read_ranging() by using scan.chan directly.

Reviewed-by: Andreas Klinger <ak@it-klinger.de>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Link: https://patch.msgid.link/20250721-iio-use-more-iio_declare_buffer_with_ts-6-v2-1-8b66e5b4e75a@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>