spi: mpc8xxx: Reduce scope of loop variables

The transmission loop starts with setting some variables, which are only
used inside the loop. Reduce the scope to the loop to make the
declaration and initialization of these variables coincide.

In the case of char_size this also always initializes the variable
immediately with the final value actually used in the loop (instead of
the placeholder value 32).

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Acked-by: Jagan Teki <jagan@amarulasolutions.com>