V4L/DVB (12780): tm6000: Improve handling of SMBus Write Byte commands followed by a read command.

SMBus Write Byte commands followed by a read command on the same I2C
address are not sent out to the chip as they wouldn't have any effect.

Signed-off-by: Michel Ludwig <michel.ludwig@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>