drivers: ethernet: adin2111: fix device assignment

`k_thread_create` function has the 'adin' device passed as 1st
entry pointer. Therefore the device configuration is obtained directly
from the `dev` structure.

Adujst the code accodringly.

Fixes: 75dde83 ("drivers: ethernet: adin2111: add adin1110 support")
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>