msk: Use a void cast to mark values of dummy reads as unused.

[jhb]

Summary

Note that this required adding missing ()'s around the outermost level
of MSK_READ_MIB*. Otherwise, the void cast was only applied to the
first register read. This also meant that MSK_READ_MIB64 was pretty
broken as the uint64_t cast only applied to the first 16-bit register
read in each MSK_READ_MIB32 invocation and the 32-bit shift was only
applied to the second register read of the pair.

Reported by: GCC -Wunused-value

[imp]

though not sure the value of the void casts...

[jhb]

(void) casts is what I used to quiet -Wunused-but-set-variable warnings in clang for cases like this where we want to force a register read without using the result. OTOH, reading the "spare" registers in msk_stats_update might be dubious and we'd be better off removing those perhaps?

[jhb]

• Removed reading unused spare registers in a separate commit.

[imp]

Sounds good (both the explanation and the splitting up of the commit)