UHDDevice: setRxGain on chan 0 when using multi-arfcn
UHDDevice: setRxGain on chan 0 when using multi-arfcn

When using multi-arfcn feature, several logical channels (arfcn) are multiplxed
into one physical transceiver, as can be seen in
uhd_device::set_channels.

As a result, when multi-arfcn is enabled some properties are actually
shared for those logical channels, and internally mapped to the first
(only existing) channel, and per-channel internal array variables are allocated
accordingly (size() == 1).

When setting RxGain, we need to set the correct existing physical
channel. Same check is done in getRxGain, and then we apply the RxGain correctly and
we avoid outputing an error "Requested non-existent channel" immediatelly after.

Change-Id: I5b02bb1ef6450dc48be7b8058d96a5691847d3cc

osmocom/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,osmocom/osmo-trx,osmocom/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx,ttsou/osmo-trx