spi: mpc8xxx: Fix if check

spi: mpc8xxx: Fix if check

Decreasing the bit length and increasing the write data pointer should
be done when there are more than 32 bit of data, not 16 bit.

This did not produce incorrect behavior, because the only time where the
two checks produce different outcomes is the case of 16 < bitlen < 32,
and in this case the subsequent transmission is the last one regardless,
hence the additional bit length decrease and write data pointer increase
has no effect anyway.

Still, the correct check is the check for "bitlen > 32", so correct this
behavior.

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