net: recv(): return -EAGAIN instead of 0 when no cleanup is expected

Some drivers do not behave properly when free_pkt() is called with a
length of zero. It is an issue I observed when developing the lwIP
series [1] (see "QEMU CI tests for r2dplus_i82557c, r2dplus_rtl8139"
in the change log) and which I fixed incorrectly by not calling
free_pkt() when recv() returns 0. That turned out to be wrong for two
reasons:

1. The DM documentation [2] clearly requires it:

  "The **recv** function polls for availability of a new packet. [...]
   If there is an error [...], return 0 if you require the packet to
   be cleaned up normally, or a negative error code otherwise (cleanup
   not necessary or already done).

   If **free_pkt** is defined, U-Boot will call it after a received
   packet has been processed [...]. free_pkt() will be called after
   recv(), for the same packet [...]"

2. The imx8mp_evk platform will fail with OOM errors if free_pkt() is
   not called after recv() returns 0:

   u-boot=> tftp 192.168.0.16:50M
   Using ethernet@30be0000 device
   TFTP from server 192.168.0.16; our IP address is 192.168.0.48
   Filename '50M'.
   Load address: 0x40480000
   Loading: #######################fecmxc_recv: error allocating packetp
   fecmxc_recv: error allocating packetp
   fecmxc_recv: error allocating packetp
   ...

Therefore, make recv() return -EAGAIN instead of 0 when no packet is
available and the driver doesn't expect free_pkt() to be called
subsequently.

[1] https://lists.denx.de/pipermail/u-boot/2024-August/562861.html
[2] doc/develop/driver-model/ethernet.rst

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>

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