iio: adc: at91-sama5d2: Move claim of direct mode up a level and use guard()

Move iio_device_claim_direct_mode() up one layer in the call stack,
and use guard() for scope based unlocking to simplify error handling
by allowing direct returns.

Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Link: https://patch.msgid.link/20250217141630.897334-22-jic23@kernel.org


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>