drivers/eth/mcux: Free net_buf using net_nbuf_unref

The driver was using net_buf_unref(). This technically works
ok but debugging the network buffer allocations is more
difficult if done like this.

Change-Id: If3453a49337c7a359c8af22cfdf331fccc697af5
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>