spi: mpc8xxx: Make code more readable

Introduce the to_prescale_mod and set_char_len inline functions to make
the code more readable.

Note that the added "if (bitlen > 16)" check does not change the
semantics of the current code, and hence only preserves the current
error (this will be fixed in a later patch in the series).

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