RXSYMS in software is 8 bits, but is 10 bits in hardware

[terrillmoore]

This limits us in terms of accepting large clock errors at high data dates (low spreading factors), because the RX window is specified in RX symbols, and with big clock errors you may need lots of symbols.

Changing this requires changing LMIC::rxsyms to u2_t, and improving the calculation. Then in radio.c::configLoraModem(), we need to merge the top two bits into ModemConfig2.