V4L/DVB: cx25840: Make cx25840 i2c register read transactions atomic

There was a small window between writing the cx25840 register
address over the i2c bus and reading the register contents back from the
cx25840 device that the i2c adapter lock was released.  This change ensures the
adapter lock is not released until the register read is done.

Signed-off-by: Andy Walls <awalls@md.metrocast.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>