spi-topcliff-pch: missing platform_driver_unregister() on error in pch_spi_init()

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

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>