hw/net: fsl_etsec: Reverse the RCTRL.RSF logic

Per MPC8548ERM [1] chapter 14.5.3.4.1:

When RCTRL.RSF is 1, frames less than 64 bytes are accepted upon
a DA match. But currently QEMU does the opposite. This commit
reverses the RCTRL.RSF testing logic to match the manual.

Due to the reverse of the logic, certain guests may potentially
break if they don't program eTSEC to have RCTRL.RSF bit set.
When RCTRL.RSF is 0, short frames are silently dropped, however
as of today both slirp and tap networking do not pad short frames
(e.g.: an ARP packet) to the minimum frame size of 60 bytes. So
ARP requests will be dropped, preventing the guest from becoming
visible on the network.

The same issue was reported on e1000 and vmxenet3 before, see:

commit 78aeb23 ("e1000: Pad short frames to minimum size (60 bytes)")
commit 40a87c6 ("vmxnet3: Pad short frames to minimum size (60 bytes)")

[1] https://www.nxp.com/docs/en/reference-manual/MPC8548ERM.pdf

Fixes: eb1e7c3 ("Add Enhanced Three-Speed Ethernet Controller (eTSEC)")
Signed-off-by: Bin Meng <bin.meng@windriver.com>

Message-Id: <1612923021-19746-1-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

hw/net/fsl_etsec/rings.c

@@ -502,7 +502,7 @@ ssize_t etsec_rx_ring_write(eTSEC *etsec, const uint8_t *buf, size_t size)
         return -1;
     }
 
-    if ((etsec->regs[RCTRL].value & RCTRL_RSF) && (size < 60)) {
+    if (!(etsec->regs[RCTRL].value & RCTRL_RSF) && (size < 60)) {
         /* CRC is not in the packet yet, so short frame is below 60 bytes */
         RING_DEBUG("%s: Drop short frame\n", __func__);
         return -1;