The first field of a MAC Control frame isn't the "pause" field, it's the

The first field of a MAC Control frame isn't the "pause" field, it's the
opcode; an opcode of 0x0001 means PAUSE.  Dissect the opcode field even
if it's not a PAUSE frame.

The second field of a PAUSE frame isn't the "quanta" field, it's the
pause_time field; it's in *units* of quanta.

Don't fetch fields until we need them (so we don't throw an exception if
they're missing until we actually look at them).

Clean up indentation.

svn path=/trunk/; revision=30785