spi/mpc52xx: Fix device remove function

The call sequence spi_alloc_master/spi_register_master/spi_unregister_master is
complete; it reduces the device reference count to zero, which results in
device memory being freed. An extra call to spi_master_put is unnecessary
and results in an access to free memory.

At the same time, since the call to spi_unregister_master results in device
memory being freed, it must no longer be accessed afterwards.

To fix both problems, call spi_master_get to get an extra reference to the
device, and call spi_master_put only after the last access to device data.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>