spi: spi-topcliff-pch: fix a pci_iomap() check

If pci_iomap() returns NULL, adding "PCH_ADDRESS_SIZE * plat_dev->id"
can make it non-NULL which breaks the error handling.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@linaro.org>