tools/mpy-tool.py: Fix calc of opcode size for opcodes with map caching.

Following an equivalent fix to py/bc.c.  The reason the incorrect values
for the opcode constants were not previously causing a bug is because they
were never being used: these opcodes always have qstr arguments so the part
of the code that was comparing them would never be reached.

Thanks to @malinah for finding the problem and providing the initial patch.