net: axi_emac: Change return value to -EAGAIN if RX is not ready

net: axi_emac: Change return value to -EAGAIN if RX is not ready

If there is no incoming package than axiemac_recv will return -1 which
in turn leads to printing `eth_rx: recv() returned error -1` error
message in eth_rx function. But missing a package is not an fatal error,
so return -EAGAIN in that case would be more suitable.

Signed-off-by: Maksim Kiselev <bigunclemax@gmail.com>
Link: https://lore.kernel.org/r/20230719065337.69280-1-bigunclemax@gmail.com
Signed-off-by: Michal Simek <michal.simek@amd.com>