ad9361-iiostream: Fallback to altvoltage for the channel name
ad9361-iiostream: Fallback to altvoltage for the channel name

Older versions of the kernel incorrectly used the 'altvoltage' channel type
instead of 'voltage'. Add support for this by falling back to 'altvoltage' if
the 'voltage' channel can't be found.

Signed-off-by: Lars-Peter Clausen <3318dc5ce3e4fb7c28a0b841b6801c884e1d0896@metafoo.de>

Sunderfield/libiio,sensarliar/libiio,analogdevicesinc/libiio,sensarliar/libiio,Sunderfield/libiio,Sunderfield/libiio,sensarliar/libiio,analogdevicesinc/libiio,gburca/libiio,gburca/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,gburca/libiio