Support separate RX and TX limits for AnyTone D878UV II (Plus)

[toco]

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Depending on the configured bands mode the AT D878UV II Plus has different RX and TX limits according to http://members.optuszoo.com.au/jason.reilly1/868mods.htm#SelectingBands However, the current implementation uses the same (TX) limits for both RX and TX. It would be awesome if programming channels with the RX only flag, within the range of allowed RX frequencies (but outside the TX limits) would be correctly verified.