cxgb4: Fix kernel panic while accessing sge_info

commit 479a0d13 upstream.

The sge_info debugfs collects offload queue info even when offload
capability is disabled and leads to panic.

[  144.139871] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  144.139874] CR2: 0000000000000000 CR3: 000000082d456005 CR4: 00000000001606e0
[  144.139876] Call Trace:
[  144.139887]  sge_queue_start+0x12/0x30 [cxgb4]
[  144.139897]  seq_read+0x1d4/0x3d0
[  144.139906]  full_proxy_read+0x50/0x70
[  144.139913]  vfs_read+0x89/0x140
[  144.139916]  ksys_read+0x55/0xd0
[  144.139924]  do_syscall_64+0x5b/0x1d0
[  144.139933]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  144.139936] RIP: 0033:0x7f4b01493990

Fix this crash by skipping the offload queue access in sge_qinfo when
offload capability is disabled

Signed-off-by: Herat Ramani <herat@chelsio.com>
Signed-off-by: Vishal Kulkarni <vishal@chelsio.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>