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 master device and release it after cleanup is
complete to solve the problem.
Signed-off-by: NGuenter Roeck <linux@roeck-us.net>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>