samples: rust: platform: don't call as_ref() repeatedly

In SampleDriver::probe() don't call pdev.as_ref() repeatedly, instead
introduce a dedicated &Device.

Signed-off-by: Igor Korotin <igor.korotin.linux@gmail.com>
Reviewed-by: Dirk Behme <dirk.behme@de.bosch.com>
Link: https://lore.kernel.org/r/20250620151849.281238-1-igor.korotin.linux@gmail.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>