[media] em28xx: fix+improve the register (usb control message) debugging

- avoid duplicate debugging messages in em28xx_read_reg_req_len()
- do not describe successful usb transfers in em28xx_read_reg_len()
  as "failed"
- report errors in em28xx_write_regs_req(), too
- print the usb error numbers, too

Signed-off-by: Frank Schäfer <fschaefer.oss@googlemail.com>
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>