drivers/eth/mcux: Make sure not to leak net_buf in RX

Check net_recv_data() return value and if it returns an error
then release the net_buf in order to avoid leaking it.

Coverity-CID: 158884

Change-Id: I10d411a2de4b7c7bbe2475df65d93f5b1e619679
Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>