drivers: eth_mcux: remove TX thread and allow for multiple TX buffers

remove the TX thread, as this thread can become starved and unable to
reclaim TX buffers from the hardware. Instead reclaim buffers in ISR.
Change eth_tx function to first take from the tx_buf_sem, so that multiple
TX buffer descriptors can be used effectively.

Performance change (as tested with iperf on RT1050)
TCP RX: 44.6Mbps->48.6Mbps
TCP TX: 38.1Mbps->40.7Mbps

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>