Reduced code size of PayloadEncrypt

[mvds00]

No description provided.

[mluis1]

Few comments:

1. Why change the size function parameter type?
   We think that the uint16_t type must be kept. The payload maximum size is 256 (1 byte)
2. Please apply the coding conventions to the line
   for( uint8_t i = 0; i < (size>=16?16:size); i++ )
   By
   for( uint8_t i = 0; i < ( ( size >= 16 ) ? 16 : size ); i++ )

[mvds00]

I changed the coding style.

The logic as it stands now, after this commit, needs size to be signed as it may drop below 0 at the last iteration. As I don't want to break existing code (that I possibly don't know of) I didn't want to decrease the range of the size parameter, so I made it 32 bit signed. I changed it into 16 bit signed, which should be perfectly fine indeed.