media: tm6000: cleanup trival coding style issues

- Delete seven error messages for a failed memory allocation
- Adjust seven checks for null pointers
- Use common error handling code in tm6000_usb_probe()
- Adjust jump targets so that the function "kfree" will be always called
  with a non-null pointer.
- Delete an initialisation for the local variable "dev"
  which became unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>