sfc/siena: fix TX channel offset when using legacy interrupts

As in previous commit for sfc, fix TX channels offset when
efx_siena_separate_tx_channels is false (the default)

Fixes: 25bde571

 ("sfc/siena: fix wrong tx channel offset with efx_separate_tx_channels")
Reported-by: Tianhao Zhao <tizhao@redhat.com>
Signed-off-by: Íñigo Huguet <ihuguet@redhat.com>
Link: https://lore.kernel.org/r/20220915141653.15504-1-ihuguet@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>