[media] rtl2832_sdr: enhance sample rate debug calculation precision

Sample rate calculation gives a little bit too large results because
in real life there was around one milliseconds (~one usb packet) too
much data for given time. Calculate time more accurate in order to
provide better results.

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