spi/mxs: 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. The remove function accesses the freed memory after
the call to spi_unregister_master(), _and_ it calls spi_master_put on the freed
memory.

Acquire a reference to the SPI master device and release it after cleanup is
complete (with the existing spi_master_put) to solve the problem.

Also, the device subsystem ensures that the remove function is only called once,
and resets device driver data to NULL. Remove the unnecessaary calls to
platform_set_drvdata().

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>