Make fBuffer const

[NovaNekmit]

Looking through usage, {McpsReqUnconfirmed_t,McpsReqConfirmed_t,McpsReqProprietary_t}.fBuffer is used in the following way:

• extracted from either McpsReq in LoRaMacMcpsRequest
• passed through to Send
• passed through to PrepareFrame
• contents are memcpy'd to MacCtx.AppData
• Proprietary frames only: contents are memcpy'd to MacCtx.PktBuffer

It seems like it would it be possible to make fBuffer a const void* instead.

This would avoid warnings about discarding const when writting const void* to the McpsReq.