app/testpmd: fix Tx first with LSC interrupt

app/testpmd: fix Tx first with LSC interrupt

The lsc_interrupt flag is enabled by default, and when the --tx-first is
used as a command line parameter, the 32 packets can be sent out before
the link state is up, resulting in the loss of the packets, and no further
forwarding will take place.

E.g. ./build/app/testpmd -c f0 -- --tx-first --stats-period 1

When the --tx-first is used, the lsc_interrupt flag needs to be disabled,
ensuring the links are up before forwarding traffic.  Therefore, during the
parameter checking at startup, if --tx-first is used, we now warn the user,
and set lsc_interrupt to 0.

Fixes: 99cabef08855 ("app/testpmd: add parameter to start forwarding Tx first")

Signed-off-by: David Hunt <david.hunt@intel.com>
Acked-by: Pablo de Lara <pablo.de.lara.guarch@intel.com>