Transceiver52M: Use exception blocks for rate changes

UHD will throw if something goes awry in these sensitive sections,
so we should catch and shutdown gracefully. There is no recovery
if we can't set rates.

Signed-off-by: Thomas Tsou <tom@tsou.cc>
Transceiver52M: Explicitly check for USRP2 device type

Before, we assumed that non-B100 device was implicitly a USRP2
(inclusive of N200/N210). Make the check explicit so that any
unknown device will causes an error and exit.

Signed-off-by: Thomas Tsou <tom@tsou.cc>