[media] r820t: add IMR calibrate code

This code seems to calibrate I/Q phase and gain during the
device initialization.
This is done only once, and it doesn't seem to be needed to
happen after resuming.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>
Tested-by: Antti Palosaari <crope@iki.fi>