net: ip: Fix low Zperf UDP throughput with -a

When run the Zperf UDP throughput with '-a' (Asynchronous call),
zperf_work_q thread will generate and send the packets to tx_q thread.
When zperf_work_q and tx_q threads have same priority, if zperf_work_q
fails to take the semaphore of fifo_slot, it will not wait and directly
drop the net_pkt. Then it will allocate new net_pkt, repeat, and always
occupy the CPU, which leads to extreme low throughput.
For TX, when take the semaphore of fifo_slot, setting a wait time of
K_FOREVER to let other thread to process and free these packets.
For RX, when failed to take the semaphore of fifo_slot, yield the CPU
to let the thread of data path with same priority to run to reduce
dropping packets.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>