Increase maximum frequency in web interface.

[mattbk]

US LoRa frequencies can go up to 928 MHz:

• https://www.rfwireless-world.com/Tutorials/LoRa-channels-list.html
• https://lora.readthedocs.io/en/latest/#rules-and-regulations

Some T-Beams are sold on this frequency.

Mentioned previously in https://github.com/SQ9MDD/TTGO-T-Beam-LoRa-APRS/issues/44.