mprintf: Replaced internal usage of FORMAT_OFF_T and FORMAT_OFF_TU

mprintf: Replaced internal usage of FORMAT_OFF_T and FORMAT_OFF_TU

Following commit 0aafd77fa4c6f2, replaced the internal usage of
FORMAT_OFF_T and FORMAT_OFF_TU with the external versions that we
expect API programmers to use.

This negates the need for separate definitions which were subtly
different under different platforms/compilers.