fix: check recvsize before nonblocking mode in TCP recv() (AUD-004)

[occamsshavingkit]

Summary

Fixes nonblocking TCP socket returning SOCK_BUSY even when RX data is available.

Problem

In the non-IPv6 W5500 path at Ethernet/socket.c:687-692, the #else branch checked nonblocking mode before checking recvsize != 0:

#else
    if (sock_io_mode & (1 << sn)) {   // checked first
        return SOCK_BUSY;
    }
    if (recvsize != 0) {              // checked second — too late
        break;
    }
#endif

A nonblocking TCP socket returned SOCK_BUSY even when RX data was available. The RX pointer never advanced, so retries could not drain the socket and the receive buffer eventually stalled.

Fix

Swap the two checks so recvsize != 0 is evaluated first, matching the ordering already used by the IPv6 branch at lines 679-685:

#else
    if (recvsize != 0) {              // checked first
        break;
    }
    if (sock_io_mode & (1 << sn)) {   // checked second
        return SOCK_BUSY;
    }
#endif

Verification

• Multi-chip compile check: passes for all 7 _WIZCHIP_ values
• An empty nonblocking socket still returns SOCK_BUSY (the check is still present, just reordered)