fmcomms2: Add TX_LO/RX_LO frequency name fallback for older kernels

On older kernels the libiio is not able to properly recognize the TX_LO/RX_LO
suffix as part of the channel name, hence it becomes part of the property name.
Work around this by checking both attributes and use the one that is available.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>