drivers: nxp_enet: Convert RX to workqueue

Convert from multiple threads for each instance to use
one workqueue for all instances. The benefit is to save
memory and use a kernel function that already exists
for a use case like this.

Also introduce the ETH_NXP_ENET_RX_THREAD_PRIORITY kconfig,
which makes the thread priority of the workqueue configurable.

Finally, remove the code enabling the RxBufferInterrupt, since
the meaning of it isn't used currently in this driver.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>