The assembler wrongly defined _bfexts_ and _bfffo_ with the same bits
as _bfextu_; this turned all bfexts and bfffo instructions into
bfextu. Motorola's 68k Programmer's Reference Manual (1992) gives
different bits for bfexts, but still has wrong bits for bfffo. Change
bfexts and bfffo to match the 68k emulators musahi, aranym, syn68k.
The bitfield width is from 1 to 32, not 0 to 31, so move the warning
from 32 to 0. This doesn't change the warning message, so it will say
that 0 is "too big", when 0 is really too small.
