dvbsrc: improve timeout handing at locking loop

The previous approach was too much of an aproximation
by considering each polling round took roughly the step
timeout itself. This approach attempts to be more accurate
by actually measuring the elapsed time. Additionally, we
know use a 10 seconds default timeout for the tuning/locking
logic and a half a second polling step.

This patch also adds ENITR_RETRY to avoid bailing out
on temporary failures (no need to check for EAGAIN cause
we are opening the frontend on blocking mode) and starts
using it for the tuning/locking routine.
dvbsrc: drop redundant assignment