Session: Allow using sample rate from meta packet

Currently, PV only looks at the device config to determine the
sample rate. This breaks as input filters submit meta packets
containing the sample rate and provide no device config.

Example: load any file in PV using the raw binary input filter
and set the sample rate to anything but 0. PV will still show
'sa' as the time unit, indicating that it dismisses the sample
rate information sent in the meta packet.

This patch makes this work by using the sample rate if it
wasn't already set from the device config.