caif: spi: missing platform_driver_unregister() on error in cfspi_init_module()

Add the missing platform_driver_unregister() before return
from cfspi_init_module() in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>