sfc: fix TX channel offset when using legacy interrupts

In the Linux kernel, the following vulnerability has been resolved:

sfc: fix TX channel offset when using legacy interrupts

In legacy interrupt mode the tx_channel_offset was hardcoded to 1, but
that's not correct if efx_sepparate_tx_channels is false. In that case,
the offset is 0 because the tx queues are in the single existing channel
at index 0, together with the rx queue.

Without this fix, as soon as you try to send any traffic, it tries to
get the tx queues from an uninitialized channel getting these errors:
WARNING: CPU: 1 PID: 0 at drivers/net/ethernet/sfc/tx.c:540 efx_hard_start_xmit+0x12e/0x170 [sfc]
[...]
RIP: 0010:efx_hard_start_xmit+0x12e/0x170 [sfc]
[...]
Call Trace:
<IRQ>
dev_hard_start_xmit+0xd7/0x230
sch_direct_xmit+0x9f/0x360
__dev_queue_xmit+0x890/0xa40
[...]
BUG: unable to handle kernel NULL pointer dereference at 0000000000000020
[...]
RIP: 0010:efx_hard_start_xmit+0x153/0x170 [sfc]
[...]
Call Trace:
<IRQ>
dev_hard_start_xmit+0xd7/0x230
sch_direct_xmit+0x9f/0x360
__dev_queue_xmit+0x890/0xa40
[...]