spi/bcm63xx: Ensure that memory is freed only after it is no longer used

The call to spi_unregister_master() in the device remove function frees device
memory, and with it any device local data. However, device local data is still
accessed after the call to spi_unregister_master().

Acquire a reference to the SPI device and release it after cleanup is complete
to solve the problem.

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