spearmint: Fix MDX swapping (my bad)

I put bones[1] into mdxFrame_t so it's closer to MDR and MDS.
Forgot to change MDX swapping code.

Issues only affected big endian.
spearmint: Remove unneeded frame++ from R_LoadMDS/MDX

Accidently added the frame++ to MDX in last commit when copying code from MDS.

Also removed from R_LoadMDM, but code is commented out.