uhd: use default 10MHz reference configuration unless specified
uhd: use default 10MHz reference configuration unless specified

The optional GPSDO automatically modifies the reference setting,
so this patch leaves the default setting alone. Devices without
the GPSDO will default to the interal reference by the UHD
driver.

Signed-off-by: Thomas Tsou <7360a69f2d05d24df84c29f24b488ef8cc0e7ae2@vt.edu>

ttsou/kalibrate-uhd,ttsou/kalibrate-uhd