mpc85xx: tlb.c cleanups

Extract the operation to read a tlb into a function - we will need
this later to print out the tlbs, and there's no point in duplicating
the code.  Create a TSIZE_TO_BYTES macro to deal with the conversion
from the MAS field to an actual size instead of duplicating this in code.
There are a few misc other minor cleanups.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>