sfc: fix null pointer dereference in efx_hard_start_xmit

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

sfc: fix null pointer dereference in efx_hard_start_xmit

Trying to get the channel from the tx_queue variable here is wrong

because we can only be here if tx_queue is NULL, so we shouldn't

dereference it. As the above comment in the code says, this is very

unlikely to happen, but it's wrong anyway so let's fix it.

I hit this issue because of a different bug that caused tx_queue to be

NULL. If that happens, this is the error message that we get here:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000020

[...]

RIP: 0010:efx_hard_start_xmit+0x153/0x170 [sfc]