Change comments to be recognised by -Wimplicit-fallthrough

[martinling]

Fixes annoying compiler warnings such as:

../can.c: In function 'can_transmit':
../can.c:417:32: warning: this statement may fall through [-Wimplicit-fallthrough=]
  417 |                 tdhxr.data8[3] = data[7];
      |                 ~~~~~~~~~~~~~~~^~~~~~~~~
../can.c:419:9: note: here

The -Wimplicit-fallthrough warning recognises a reasonably wide selection of comments as identifying a deliberate fall through, but no break is not among them.