Modify the mmc data structure.

The new 6.0 kernel headers changed all variable length
structures from [0] to []. This causes a clang warning to
trigger, so rewrite the mmc data structures using a union
to avoid having a variable sized array in the middle of the
structure.

Test: Builds.
Change-Id: Ib1ac92c4f76af386d934f51a3c73cb8914e97624