sfc: fix considering that all channels have TX queues

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

sfc: fix considering that all channels have TX queues

Normally, all channels have RX and TX queues, but this is not true if
modparam efx_separate_tx_channels=1 is used. In that cases, some
channels only have RX queues and others only TX queues (or more
preciselly, they have them allocated, but not initialized).

Fix efx_channel_has_tx_queues to return the correct value for this case
too.

Messages shown at probe time before the fix:
sfc 0000:03:00.0 ens6f0np0: MC command 0x82 inlen 544 failed rc=-22 (raw=0) arg=0
------------[ cut here ]------------
netdevice: ens6f0np0: failed to initialise TXQ -1
WARNING: CPU: 1 PID: 626 at drivers/net/ethernet/sfc/ef10.c:2393 efx_ef10_tx_init+0x201/0x300 [sfc]
[...] stripped
RIP: 0010:efx_ef10_tx_init+0x201/0x300 [sfc]
[...] stripped
Call Trace:
efx_init_tx_queue+0xaa/0xf0 [sfc]
efx_start_channels+0x49/0x120 [sfc]
efx_start_all+0x1f8/0x430 [sfc]
efx_net_open+0x5a/0xe0 [sfc]
__dev_open+0xd0/0x190
__dev_change_flags+0x1b3/0x220
dev_change_flags+0x21/0x60
[...] stripped

Messages shown at remove time before the fix:
sfc 0000:03:00.0 ens6f0np0: failed to flush 10 queues
sfc 0000:03:00.0 ens6f0np0: failed to flush queues