kingst-la2016: reword buffer size in FPGA register access

kingst-la2016: reword buffer size in FPGA register access

Phrase communication buffer sizes in terms of FPGA register width,
instead of totalling the sizes of different bitfield width. Comment
where buffers are shared among different accesses, for awareness
during maintenance.

Introduce an "unknown register at offset 0x30" such that REG_TRIGGER
access can follow the pattern of determining the FPGA register's width.