Currently, the warning for missing device_caps gives a backtrace like so:

[<ffffffff8175c199>] dump_stack+0x45/0x57
[<ffffffff8109ad5a>] warn_slowpath_common+0x8a/0xc0
[<ffffffff8109ae8a>] warn_slowpath_null+0x1a/0x20
[<ffffffffa0237453>] v4l_querycap+0x43/0x80 [videodev]
[<ffffffffa0237734>] __video_do_ioctl+0x2a4/0x320 [videodev]
[<ffffffff812207e5>] ? do_last+0x195/0x1210
[<ffffffffa023a11e>] video_usercopy+0x22e/0x5b0 [videodev]
[<ffffffffa0237490>] ? v4l_querycap+0x80/0x80 [videodev]
[<ffffffffa023a4b5>] video_ioctl2+0x15/0x20 [videodev]
[<ffffffffa0233733>] v4l2_ioctl+0x113/0x150 [videodev]
[<ffffffff81225798>] do_vfs_ioctl+0x2f8/0x4f0
[<ffffffff8113b2d4>] ? __audit_syscall_entry+0xb4/0x110
[<ffffffff81022d7c>] ? do_audit_syscall_entry+0x6c/0x70
[<ffffffff81225a11>] SyS_ioctl+0x81/0xa0
[<ffffffff8113b526>] ? __audit_syscall_exit+0x1f6/0x2a0
[<ffffffff81763549>] system_call_fastpath+0x12/0x17

This indicates that device_caps are missing but doesn't give
much of a clue which driver is actually at fault. Improve
the warning output by showing the capabilities and the
responsible driver.
Signed-off-by: Laura Abbott <labbott@fedoraproject.org>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>