app/testpmd: fix error detection in MTU command

app/testpmd: fix error detection in MTU command

MTU is used in testpmd to set the maximum payload size for packets.
According to testpmd, the setting influence RX only.
In rte_ethdev there's no relation between MTU setting and JUMBO offload
or rx_max_pkt_len.

The previous fix in patch referenced below was meant to update the
correlated variables of max_pkt_len and JUMBO offload, but by doing so
it assumes that MTU setting can only exist when JUMBO offload supported
in the device. For example fail-safe device does supports set MTU and
doesn't support JUMBO offload, and in this case, though set MTU
succeeds, an error message is still printed since the JUMBO packet
offload is disabled.

The fix separates the two conditions to make sure the error
triggers only in case the set_mtu action actually failed.

Fixes: 150c9ac2df13 ("app/testpmd: update Rx offload after setting MTU")
Cc: stable@dpdk.org
Signed-off-by: Shy Shyman <shys@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>
Reviewed-by: Phil Yang <phil.yang@arm.com>