Invalidate cached values of MPOS/MANG when writing to ZPOS/MPOS/MANG

[neildavis]

ZPOS, MPOS & MANG are not mutually exclusive when it comes to writing.
Therefore, unless you clear the cached values for these registers when writing the others, they will report incorrect values when read.

See same issue I reported on Dan's CircuitPython version for more details: