iio: adc: ad7380: use if_not_guard for claim direct

Replace usage of iio_device_claim_direct_scoped() with
if_not_guard().

This makes fewer lines of code, less indentation, avoids having the
error return statement in the macro args, and avoids needing to use
unreachable().

Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20241024-cleanup-if_not_guard-v2-1-1bef98c9fd2e@baylibre.com