mmc: sdhci-pci: Fix possibility of chip->fixes being null

commit 945be38c upstream.

It is possible for chip->fixes to be null.  Check before dereferencing it.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <chris@printf.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>