cxgb4: Fix trace observed while dumping clip_tbl

Handle clip_tbl debugfs entry, when clip_tbl isn't allocated.
In commit b5a02f503caa0837 ("cxgb4: Update ipv6 address handling api") wrong
argument was passed for single_open for clip_tbl debugfs entry, which led to
below trace. Fixing it.

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call Trace:
 [<ffffffffa073c606>] clip_tbl_open+0x16/0x30 [cxgb4]
 [<ffffffff8119e2fa>] do_dentry_open+0x21a/0x370
 [<ffffffff8119e499>] vfs_open+0x49/0x50
 [<ffffffff811b0d0e>] do_last+0x21e/0x800
 [<ffffffff811b1382>] path_openat+0x92/0x470
 [<ffffffff8110569f>] ? rb_reserve_next_event+0xaf/0x380
 [<ffffffff8110569f>] ? rb_reserve_next_event+0xaf/0x380
 [<ffffffff811b189a>] do_filp_open+0x4a/0xa0
 [<ffffffff811bdc5d>] ? __alloc_fd+0xcd/0x140
 [<ffffffff8119fa4a>] do_sys_open+0x11a/0x230
 [<ffffffff8101219f>] ? syscall_trace_enter_phase2+0xaf/0x1b0
 [<ffffffff8119fb9e>] SyS_open+0x1e/0x20
 [<ffffffff815bf6f0>] tracesys_phase2+0xd4/0xd9
Code: 89 e5 66 66 66 66 90 48 8b 47 e0 48 8b 40 30 48 8b 40 58 c9 c3 66 0f 1f
84 00 00 00 00 00 55 48 89 e5 66 66 66 66 90 48 8b 47 e0 <48> 8b 40 58 c9 c3 66
66 66 66 2e 0f 1f 84 00 00 00 00 00 55 48
RIP  [<ffffffff8120898d>] PDE_DATA+0xd/0x20
 RSP <ffff8800b08c3c48>
CR2: 0000000000000058

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Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>