examples/l3fwd: fix scalar LPM

examples/l3fwd: fix scalar LPM

The lpm_process_event_pkt() can either process a packet using an
architecture specific (defined for X86/SSE, ARM/Neon and PPC64/Altivec)
path or a scalar one. The choice is however done using an ifdef
pre-processor macro. Because of that the scalar version was apparently
not widely exercised/compiled.
Due to some copy/paste errors, the scalar logic in
lpm_process_event_pkt() retained a "continue" statement where it should
utilize rfc1812_process() and return the port/BAD_PORT.

Fixes: 99fc91d18082 ("examples/l3fwd: add event lpm main loop")
Signed-off-by: Stanislaw Kardach <kda@semihalf.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>