[media] tda8290: Make all read operations atomic

Read operations should be preceeded by a write operation. However,
nothing prevents that an I2C operation could happen between the two
transactions.

To avoid that problem, use an unique I2C transfer for both parts of
the I2C transaction.

Cc: Michael Krufky <mkrufky@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>