drivers: ethernet: adin2111: oa: fix thread termination

If OA read fails, then the essential thread is terminated.
Correct the behavior so that the driver re-tries the read and
continues to work.

Refactor offload thread OA and Generic SPI parts into
if/else statement.

Add missing `is_adin2111` to OA port 2 RX status check.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>