uhd: Set minimum supported version to 3.9.0

Versions of UHD prior to 3.9.0 are no longer supported.

Rather then backport and ifdef UHD version specific API and
behavioral changes, set minimum support to the current LTS
release, which is 3.9.0.

Change-Id: Id7d15b52cd4e45f1d856a6ef3a84832a28f2dd04
Signed-off-by: Tom Tsou <tom.tsou@ettus.com>
2 files changed