spi: mpc8xxx: Document LEN setting better

Instead of having a table right before the code implementing the length
setting for documentation, have inline comments for the if branches
actually implementing the length setting described table's entries
(which is readable thanks to the set_char_len function).

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