[LoRaWAN] Fix session restore throwing unnecessary error

[StevenCellist]

If a session is stored, the value returned by beginOTAA(), beginABP() and restore() is positive, returning the active mode (OTAA / ABP). This was not handled correctly. Plus reading this mode value from NVM was done in only 1 byte width. From #935 it appears this was accidentally not a relevant bug, fortunately.

[jerryneedell]

I hope you don't mind my commenting here -- I don't see the fix you gave me being implemented at lines 114/115 in LoraWAN.cpp
Just curious if one of the other changes made it unnecessary?

if(this->band->bandType == RADIOLIB_LORAWAN_BAND_DYNAMIC) {
  // 114: mod->hal->...
  // 115: execMacCommand(...)
}```

[StevenCellist]

I don't mind! You encountered two unrelated problems; this PR addresses the first, I'll send in a second one later that'll also solve the other small bug I found when looking at your logs.

[jerryneedell]

Great! I just wanted to make sure I was understanding things. Thank you for your quick attention to this and your patience.

[jgromes]

@StevenCellist merged, thanks for keeping an eye out for LoRaWAN-related issues :)

[StevenCellist]

No problem! Trying my best to support the development. If I end up missing something, feel free to tag me.