spi: Fix WARN when removing spi-fsl-spi module

If CPM mode is not used, the fsl_dummy_rx variable is never allocated.  When
the cleanup attempts to free it, the reference count is zero and a WARN is
generated.  The same CPM mode check used in the initialize is applied to the
free as well.

Tested on 2.6.33 with the previous spi_mpc8xxx driver.  The renamed
spi-fsl-spi driver looks to have the same problem.

Signed-off-by: Jeff Harris <jeff_harris@kentrox.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>