V4L/DVB (12821): tm6000: update USB request names and clean up i2c routine

Update the descriptions of the USB request types so that they match what
we now know they do.

Rework the i2c_xfer function so that it is more explicit what sort of I2C
transfers it is that the tm6000 can't perform.

Signed-off-by: Chris Pascoe <c.pascoe@itee.uq.edu.au>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>